Patent Description:
A supported catalyst obtained by supporting a catalyst metal on a carrier is used in many fields and is used as a catalyst for exhaust gas purification, liquid-phase chemical synthesis reaction, gas-phase chemical synthesis reaction, fuel cell reaction, etc..

As to such a supported catalyst, it is known that the size of the catalyst metal particles supported on a carrier is important. In this connection, for example, Patent Document <NUM> has proposed a supported catalyst in which catalyst metal particles having a size of <NUM> to <NUM> are supported on a carrier of alumina, silica, titania, zirconia or a combination thereof. In addition, Patent Document <NUM> has proposed a catalyst for nitrogen oxide, in which copper ion is supported on zeolite by ion exchange. Furthermore, Patent Document <NUM> has proposed a technique where palladium supported on zeolite by ion exchange is dried and then clustered by a reduction treatment to prepare a cluster-supporting catalyst, and the cluster-supporting catalyst is used for a coupling reaction, etc..

Incidentally, Patent Document <NUM> finds that zeolite structure collapses by migration of aluminum atom from the zeolite structure in a high temperature atmosphere containing water vapor such as an exhaust gas atmosphere, and proposes improving heat resistance of zeolite as a catalyst carrier by substituting at least part of aluminum atom in aluminosilicate zeolite with boron atom, calcium atom, indium atom, etc..

Further, in Patent Document <NUM>, which is not disclosed at the filing date of the priority application of the present application, discloses a cluster-supporting catalyst comprising porous carrier particles having acid sites such as zeolite, and catalyst metal clusters supported within the pores of the porous carrier particles; and a method for producing the same.

Incidentally, it is known that boron-substituted zeolite, i.e. zeolite having a structure in which at least part of aluminum atom constituting aluminosilicate zeolite is substituted with boron atom, can be obtain by the method disclosed in Non-Patent Document <NUM>.

<CIT>
discloses catalyst metal clusters (noble metals, Pt) supported on and in a boron substituted zeolite.

Although various supported catalysts have been proposed as described above, a supported catalyst having further improved catalytic activity is demanded. In this connection, the cluster-supporting catalyst described in Patent Document <NUM> can exhibit reaction properties different from conventional catalysts, but there is room for improvement in regard to the heat resistance. Among others, in the field of exhaust gas purification, a supported catalyst having an improved low-temperature activity for nitrogen oxide (NOx) reduction and/or carbon monoxide (CO) oxidation after thermal endurance is demanded.

Accordingly, the present invention provides a cluster-supporting catalyst having improved heat resistance, and a production method thereof, as defined in the claims.

The present inventors have found that a specific cluster-supporting catalyst has improved heat resistance, and arrived at the present invention described below.

According to the cluster-supporting catalyst of the present invention, an improved catalytic activity can be provided even after subjected to endurance treatment, especially hydrothermal endurance treatment.

The catalyst metal clusters can having a positive charge, and be supported on the acid sites within the pores of the boron-substitute zeolite particles through an electrostatic interaction. The cluster-supporting catalyst of the present invention can be produced by the method of the present invention described below.

The cluster-supporting catalyst of the present invention has excellent low-temperature activity even after thermal endurance treatment, especially even after hydrothermal endurance treatment which is a thermal endurance treatment in an atmosphere containing water vapor, and this means that the cluster in the boron-substitute zeolite particles is stably maintained despite being subjected to thermal endurance treatment.

Although not to be bound by theory, the heat resistance, especially hydrothermal resistance of the cluster-supporting catalyst of the present invention is believed to be provided by the facts that catalyst metal is supported as clusters rather than as ions on the zeolite particles, and the zeolite particles supporting the clusters are boron-substitute zeolite particles, and thus interaction between the clusters and negative charge of the acid sites of the zeolite particles is delocalized, and this prevent collapse of the skeleton structure of the zeolite particles.

Particularly, in the case where catalyst metal supported in the pores of boron-substituted zeolite particles is in a state of cluster having positive charge, it is believed that the positive charge is delocalized among multiple atoms constituting cluster, and thus catalyst metal cluster delocally bonds to multiple acid cites in the boron-substituted zeolite particle, and as a result, relatively does not distort skeleton structure of the zeolite. On the other hand, in the case where catalyst metal supported in the pores of boron-substituted zeolite particles is in a state of single atom ion rather than cluster, it is believe that catalyst metal ion has at least one positive charge per one atom, and thus strongly bonds to one acid site having negative charge in the boron-substituted zeolite particle, and as a result, this strong bond distorts skeleton structure of the zeolite, and expedites collapse of zeolite structure, especially under hydrothermal endurance treatment condition.

Further, specifically, it is believed that, since the zeolite particles used in the present invention are boron-substituted zeolite particles in which at least part of aluminum atoms therein are substituted with boron atoms, rather than common zeolite particles, i.e. aluminosilicate zeolite particles, negative charge, which forms acid site, is also delocalized, and thereby expedites delocalized bond between cluster and acid site. Incidentally, the delocalization of negative charge is believed to be based on the fact that boron has a similar electronegativity to silicon in comparison with aluminum.

Also although not to be bound by theory, the heat stability of the clusters-supporting catalyst of the present invention, which is produced by the method of the present invention described below, are believed to be attributable to the fact that, for example, the clusters having a positive charge are stably fixed to the acid sites having a negative charge, and/or the clusters supported within the pores have a relatively uniform size.

Incidentally, the "cluster" is generally defined as an aggregate of up to several hundred chemical species, but in the present invention, the "cluster-supporting catalyst" means a catalyst in which a fine catalyst metal including a clusters are supported on the carrier particle.

The cluster-supporting catalyst of the present invention can be preferably used, for example, as an exhaust gas purification catalyst, a catalyst for liquid-phase compound synthesis reaction, a catalyst for gas-phase synthesis reaction or a catalyst for fuel cell reaction, particularly, as an exhaust gas purification catalyst.

The catalyst metal constituting the catalyst metal clusters is rhodium.

The cluster-supporting catalyst obtained by the method of the present invention can stably maintain the catalytic activity of the catalyst metal, and the catalytic activity of the catalyst metal can thereby be provided while reducing the use amount of the catalyst metal.

Incidentally, in view of catalytic activity, the catalyst metal is preferably particles having a fine particle diameter. As described below regarding the method of the present invention for judging the metal particle size, the catalyst metal particles can be confirmed to have a fine particle diameter by utilizing a phenomenon that when the metal particles have a particle diameter of <NUM> or less, particularly, when the metal particles are of a cluster size, the metal particles emits fluorescence upon irradiation with excitation light.

The boron-substituted zeolite particles are silicalite type (code: MFI).

In the present invention, the boron-substituted zeolite particles can have a structure in which at least part of aluminum atoms constituting aluminosilicate zeolite is substituted with boron atoms. For example, the substitute ratio of aluminum atom with boron atom is <NUM> atom% or more, <NUM> atom% or more, <NUM> atom% or more, <NUM> atom% or more, <NUM> atom% or more, or <NUM> atom% or more, and especially the boron-substituted zeolite particles can be ones which substantially do not contain aluminum atoms.

The catalyst device of the present invention includes the cluster-supporting catalyst of the present invention, a substrate supporting the cluster-supporting catalyst, and optionally, a vessel holding the substrate.

In the catalyst device of the present invention, a honeycomb substrate, particularly, a cordierite-made honeycomb substrate, can be used as the substrate. Furthermore, in the catalyst device of the present invention, a vessel made of a metal such as stainless steel can be used as the optional vessel.

In the method of the present invention for producing a cluster-supporting catalyst, a cluster-supporting catalyst, particularly, the cluster-supporting catalyst of the present invention, including boron-substituted zeolite particles having acid sites, and catalyst metal clusters supported within the pores of the boron-substituted zeolite particles, is produced.

This method includes: providing a dispersion liquid containing a dispersion medium and boron-substituted zeolite particles dispersed in the dispersion medium; and in the dispersion liquid, forming catalyst metal clusters having a positive charge, and supporting the catalyst metal clusters on the acid sites within the pores of the boron-substituted zeolite particles through an electrostatic interaction. The method may further include drying and firing the boron-substituted zeolite particles having supported thereon the catalyst metal clusters.

According to the method of the present invention, in a dispersion liquid in which boron-substituted zeolite particles is present, catalyst metal clusters are formed and the formed catalyst metal clusters are supported within the pores of the boron-substituted zeolite particles, whereby catalyst metal clusters having a controlled size, particularly, catalyst metal clusters having a relatively uniform size, can be supported within the pores of the boron-substituted zeolite particles. The size of the catalyst metal clusters can be controlled by adjusting the conditions for forming the catalyst metal clusters in the dispersion liquid.

In the method of the present invention, catalyst metal clusters can be supported within the pores of the boron-substituted zeolite particles through an electrostatic interaction between catalyst metal clusters and acid sites of the boron-substituted zeolite particles.

The electrostatic interaction enables the catalyst metal clusters having a positive charge to be supported on the acid sites within the pores of the boron-substituted zeolite particles having a negative charge.

The catalyst metal clusters having a positive charge can be supported on the acid sites within the pores of the boron-substituted zeolite particles having a negative charge. Accordingly, the boron-substituted zeolite particles preferably have a relatively small zeta potential and may have a zeta potential of, for example, -<NUM> mV or less, -<NUM> mV or less, -<NUM> mV or less, or -<NUM> mV or less. For the same reason, the boron-substituted zeolite particles preferably has a relatively large number of acid sites, i.e., has a relatively small Si/(Al and B) ratio, and may have an Si/(Al and B) ratio of, for example, <NUM> or less, <NUM> or less, <NUM> or less, or <NUM> or less.

In the method of the present invention, the dispersion liquid can be provided by pulverizing the boron-substituted zeolite particles and dispersing the pulverized boron-substituted zeolite particles in the dispersion medium.

In this case, since the boron-substituted zeolite particles are previously pulverized, the catalyst metal clusters can be accelerated to be supported within the pores of the boron-substituted zeolite particles. Incidentally, such a pulverized boron-substituted zeolite particles have sometimes become amorphous, and the boron-substituted zeolite particles may therefore be recrystallized, if desired, by annealing before or after the catalyst metal clusters are supported.

Accordingly, as the dispersion medium, a dispersion medium having a lower dielectric constant than that of water (dielectric constant: <NUM>), acetone (dielectric constant: <NUM>), is used as the dispersion medium.

The catalyst metal cluster, particularly, the catalyst metal clusters having a positive charge, are formed by laser ablation in liquid.

A pulsed laser is converged and introduced into this solution. According to this introduction, a plasma is generated in the laser focus region to produce various chemical species (a metal ion from which a ligand is removed, a plasma metal ion produced after electron detachment of a negative metal ion source, etc.) from the metal ion source, and furthermore, a neutral metal atom forms an aggregate with a positive metal ion to produce a positively charged metal clusters. The positively charged metal cluster produced in this way is supported on the acid sites of zeolite through an electrostatic interaction.

The present invention is described in greater detail below by referring to Examples, but needless to say, the scope of the present invention is not limited by these Examples.

Rhodium cluster-supporting boron-substituted zeolite were produced by a laser abrasion method of rhodium target in acetone.

As illustrated in <FIG>, acetone <NUM> as a dispersion medium having dispersed therein carrier particles (not shown) was put in a vessel <NUM>, a plate <NUM> of rhodium was placed in acetone <NUM>, the plate <NUM> of rhodium in acetone <NUM> was irradiated with a laser <NUM> through a lens <NUM>, and a rhodium cluster <NUM> was formed in the acetone by laser ablation. The thus-formed rhodium cluster <NUM> took on a positive charge and therefore, as illustrated in <FIG>, was electrically drawn to sites having a negative charge, i.e., acid sites, of the carrier particles of zeolite carrier particles <NUM>, and supported there.

The laser light was the basic wave (<NUM>,<NUM>, <NUM>) of a Nd:YAG laser, and the intensity thereof was <NUM> W.

The carrier particles supporting the clusters were taken out from the acetone, dried at about <NUM> for about <NUM> hour, and fired at <NUM> for <NUM> hours to obtain the catalyst of Example <NUM> in which rhodium clusters were supported on the boron-substituted zeolite particles.

In Example <NUM>, the carrier particles and the laser irradiation time were as follows:.

Incidentally, the boron-substituted zeolite particles used in Example <NUM> were prepared by a microwave heating method according to the method of Non-Patent Document <NUM> above.

The catalyst of Comparative Example <NUM>, in which rhodium clusters are supported on the common silica-alumina zeolite particles (MFI-type zeolite (Al-MFI) (Si/Al ratio: <NUM>)), was obtained in the same manner as in Example <NUM> except that non-substituted common silica-alumina zeolite particles were used in place of boron-substituted zeolite particles. The amount of rhodium supported was <NUM> mass%.

The catalyst of Comparative Example <NUM>, in which rhodium ions are supported on boron-substituted zeolite particles, was obtained by supporting rhodium on the boron-substituted zeolite particles used in Example <NUM> through ion exchange method. The amount of rhodium supported was <NUM> mass%.

More specifically, in this comparative example, the catalyst of Comparative Example <NUM> was obtained by dispersing B-MFI in a <NUM> rhodium nitrate (III) aqueous solution, stirring the resulted solution for <NUM> hours, taking the B-MFI from the aqueous solution, drying the thus obtained B-MFI for <NUM> hour at the temperature of about <NUM>, and then firing the thus obtained B-MFI for <NUM> hours at the temperature of <NUM>.

The catalyst of Comparative Example <NUM>, in which rhodium ions are supported on common silica-alumina zeolite particles, was obtained by supporting rhodium on the common silica-alumina zeolite particles used in Comparative Example <NUM> through ion exchange method similar to that used in Comparative Example <NUM>. The amount of rhodium supported was <NUM> mass%.

For the rhodium cluster-supporting boron-substituted zeolite catalyst of Example <NUM>, the rhodium cluster-supporting silica-alumina zeolite catalyst of Comparative Example <NUM>, the rhodium ion-exchanged boron-substituted zeolite catalyst of Comparative Example <NUM>, and the rhodium ion-exchanged silica-alumina zeolite catalyst of Comparative Example <NUM>, hydrothermal endurance treatment was conducted at the temperature of <NUM> (Comparative Example <NUM>) or <NUM> (Example <NUM>, and Comparative Examples <NUM> and <NUM>) in the atmosphere having <NUM> % water vapor pressure.

For samples subjected to the above hydrothermal endurance treatment, CO <NUM>% purification temperature were evaluated through CO oxidation reaction in a dry reaction gas atmosphere of <NUM> and <NUM> atm. Incidentally, the evaluation was conducted such that the total amount of rhodium supported on zeolite became the same.

The reaction gas composition was CO: <NUM>%, O2: <NUM>%, and He: balance. The space velocity was <NUM>,<NUM>.

The evaluation result is shown in <FIG>. As shown the figure, the rhodium cluster-supporting boron-substituted zeolite catalyst of Example <NUM> has excellent catalytic property before the endurance treatment, and degradation of the catalytic property after the hydrothermal endurance treatment was small.

The catalytic properties of rhodium-supporting zeolites of Comparative Examples <NUM> and <NUM>, in which common silica-alumina zeolite (Al-MFI) was used in place of boron-substituted zeolite (B-MFI), were degraded by hydrothermal endurance treatment, regardless whether rhodium is supported on zeolite as clusters (Comparative Example <NUM>) or as ion (Comparative Example <NUM>). This is believed to be because silica-alumina zeolite is less heat resistive, and thus the zeolite structure thereof is deformed by water vapor used in the hydrothermal endurance treatment.

Incidentally, the heat endurance treatment was conducted at <NUM> for the catalysts of Example <NUM>, and Comparative Examples <NUM> and <NUM>, while the heat endurance treatment was conducted at <NUM> for the catalyst of Comparative Example <NUM>. However, the catalyst of Comparative Example <NUM> more degraded than the catalyst of other examples. Therefore, it is understood that the catalyst of Comparative Example <NUM> would have more remarkably degraded if the hydrothermal endurance treatment at <NUM> is used for the catalyst of Comparative Example <NUM>.

Before the hydrothermal endurance treatment, the catalyst activity of rhodium-ion-exchanged boron-substituted zeolite catalyst of Comparative Example <NUM> was lower than the catalyst activity of the catalyst of Example <NUM>, the catalyst of Comparative Example <NUM> using the boron-substituted zeolite as in the case of Example <NUM> while supporting rhodium as ions rather than clusters. Further, after the hydrothermal endurance treatment at <NUM>, the catalyst activity of the catalyst of Comparative Example <NUM> more degraded than that of Example <NUM>. This is believed to be because the bonding between the rhodium ions and the acid site of the rhodium-ion-exchanged boron-substituted zeolite is strong, and the strong bonding deforms the skeleton structure of the zeolite and then expedite the collapse of the zeolite structure.

In the following Reference Example and Comparative Reference Examples, which corresponds to Examples and Comparative Examples of Reference <NUM> above, it is shown that metal clusters can be supported in the pore of the zeolite particles by the method of the present invention, and that the thus produced catalyst supporting catalyst metal cluster in the pores thereof exhibit superior exhaust gas purifying activity.

In Reference Examples <NUM> and <NUM> and Reference Comparative Example <NUM>, gold clusters were formed by a laser ablation method of gold target in acetone, and the gold clusters were supported on carrier particle to prepare a supported gold-cluster catalyst. The catalysts obtained in Reference Examples <NUM> and <NUM> and Reference Comparative Example <NUM> were evaluated for the fluorescence spectrum. The catalyst of Reference Example <NUM> was evaluated also for the supporting rate in pore.

As illustrated in <FIG>, acetone <NUM> as a dispersion medium having dispersed therein carrier particles (not shown) was put in a vessel <NUM>, a plate <NUM> of gold was placed in acetone <NUM>, the plate <NUM> of gold in acetone <NUM> was irradiated with a laser <NUM> through a lens <NUM>, and a gold cluster <NUM> was formed in the acetone by laser ablation. The thus-formed gold cluster <NUM> took on a positive charge and therefore, as illustrated in <FIG>, was electrically drawn to sites having a negative charge, i.e., acid sites, of the carrier particles of zeolite carrier particles <NUM>, and supported there.

The carrier particles supporting the clusters were taken out from the acetone, dried at about <NUM> for about <NUM> hour, and fired at <NUM> for <NUM> hours to obtain the supported gold-cluster catalyst of Reference Example <NUM>.

In Reference Example <NUM>, the carrier particles and the laser irradiation time were as follows:.

The supported gold-cluster catalyst of Reference Example <NUM> was obtained in the same manner as in Reference Example <NUM> except that the carrier particles and the laser irradiation time were as follows:.

Incidentally, since the ablation efficiency differs depending on the surface state of the carrier particles or the plate of gold, in Reference Example <NUM> and Reference Comparative Example <NUM>, the laser ablation time was adjusted to provide the same amount of ablation of gold as in Reference Example <NUM>. The amount of ablation of gold was judged from the change in color of the dispersion medium.

The supported gold-cluster catalyst of Reference Comparative Example <NUM> was obtained in the same manner as in Reference Example <NUM> except that the carrier particles and the laser irradiation time were as follows:.

With respect to the supported gold-cluster catalysts of Reference Examples <NUM> and <NUM> and Reference Comparative Example <NUM>, the measurement of fluorescence spectrum (excitation wavelength: <NUM>) was performed. <FIG> depicts a graph illustrating the evaluation results of fluorescence spectrum normalized to the intensity per <NUM> of gold. In <FIG>, the result as to Reference Example <NUM> is indicated by the spectrum (i), the result as to Reference Example <NUM> is indicated by the spectrum (ii), and the result as to Reference Comparative Example <NUM> is indicated by the spectrum (iii).

In <FIG>, the fluorescence signal at near <NUM> is a spectrum in which fluorescent emissions from the gold cluster of about <NUM>-mer were overlapped. Accordingly, <FIG> indicates that in the supported gold-cluster catalysts of Reference Examples <NUM> and <NUM>, particularly, in the supported gold-cluster catalyst of Reference Example <NUM>, a relatively large amount of a gold cluster around <NUM>-mer is supported on the carrier particle.

In <FIG>, for the sake of examination, based on the spectra of <FIG>, the result as to Reference Example <NUM> (spectrum (i)) is depicted at <NUM>-fold magnification, the result as to Reference Example <NUM> (spectrum (ii)) is depicted at <NUM>-fold magnification, and the result as to Reference Comparative Example <NUM> (spectrum (iii)) is depicted at <NUM>-fold magnification.

Compared with the results (spectra (i) and (ii)) as to Reference Examples <NUM> and <NUM> where the gold clusters were supported on zeolite, the result (spectrum (iii)) as to Reference Comparative Example <NUM> where the gold clusters were supported on fumed silica is shifted to the long wavelength side. Thus suggests that the particle diameter of the gold cluster supported on fumed silica of Reference Comparative Example <NUM> is larger than that of the gold cluster supported on zeolite of Reference Examples <NUM> and <NUM>. Incidentally, the peak near <NUM> is derived from Mie scattering by nanoparticles attached to the carrier particle surface simultaneously with the clusters.

In Reference Examples <NUM> to <NUM> and Reference Comparative Example <NUM>, a gold clusters were formed by using a gold target. In this connection, as to the following metals, it was confirmed that clusters of the metal can be formed in the same manner as in Reference Example <NUM> by the method of laser ablation in liquid using the metal as the target:
Aluminum, silicon, titanium vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium, germanium zirconium niobium, silver, rhodium ruthenium, palladium, indium, tin, tantalum, tungsten, iridium, platinum, and cerium.

It was also confirmed that out of these metal clusters, as to copper, silver, rhodium, ruthenium palladium and platinum, fluorescence is observed upon irradiation with excitation light. In addition, it was confirmed that out of these metal clusters, as to copper, silver, rhodium, ruthenium and platinum, fluorescence is observed upon irradiation with excitation light also when clusters formed by the ion exchange-reduction method are supported on a zeolite carrier particle.

In Reference Example <NUM>, a supported copper-cluster catalyst in which copper clusters are supported on a zeolite carrier particles was prepared in the same manner as in Reference Example <NUM> except that a copper target was used in place of the gold target and a ZSM-<NUM> zeolite carrier particles (MFI) (Si/Al ratio: <NUM>) were used as the zeolite carrier particle. The obtained catalyst was evaluated for the fluorescence spectrum.

Unlike the gold, the copper is oxidized in air, and the copper cluster immediately after preparation was in the oxide state. Accordingly, the supported copper-cluster catalyst immediately after preparation did not emit fluorescence.

Then, the obtained supported copper-cluster catalyst was heated at <NUM> for <NUM> hours in a hydrogen atmosphere, effecting a reduction treatment, and thereafter evaluated for the fluorescence intensity. Consequently, the supported copper-cluster catalyst subjected to a reduction treatment exhibited fluorescence. The result of the fluorescence intensity evaluation (excitation wavelength: <NUM>) is illustrated as the spectrum (i) in <FIG>. In this spectrum (i), the fluorescence of <NUM> to <NUM> corresponds to the already-reported fluorescence signals of <NUM>-mer and <NUM>-mer of copper.

Subsequently, the supported copper-cluster catalyst was left standing overnight in an air atmosphere, effecting an oxidation treatment, and again evaluated for the fluorescence intensity. Consequently, the supported copper-cluster catalyst left standing in an air atmosphere exhibited fluorescence, though the intensity was weak compared with that before the treatment in an air atmosphere. The result of fluorescence intensity evaluation is illustrated as the spectrum (ii) in <FIG>.

Subsequently, the supported copper-cluster catalyst left standing in an air atmosphere was again subjected to the above-described reduction treatment and again evaluated for the fluorescence intensity. Consequently, the supported copper-cluster catalyst again subjected to the reduction treatment exhibited the same fluorescence as that before the treatment in an air atmosphere. The result of the fluorescence intensity evaluation is illustrated as the spectrum (iii) in <FIG>.

The supported copper-cluster catalyst after performing oxidation treatment and reduction treatment thus exhibited the same fluorescence as that before these treatments, and this suggests that the copper clusters are held within the pores of the zeolite carrier particles and in turn, a change such as aggregation of the copper clusters are not caused by these treatments.

In Reference Example <NUM> and Reference Comparative Example <NUM>, the rhodium cluster-supporting catalyst (Reference Example <NUM>) and a commercially available exhaust gas purification catalyst (Reference Comparative Example <NUM>) were evaluated for the catalytic activity.

Specifically, Reference Example <NUM> and Reference Comparative Example <NUM> were conducted as follows.

In Reference Example <NUM>, rhodium clusters were supported on a zeolite carrier particles in the same manner as in Reference Example <NUM> except that a rhodium target was used in place of the gold target and a beta-type zeolite carrier particles (BEA) (Si/Al ratio: <NUM>) were used as the zeolite carrier particle.

The temperature (TCO(<NUM>%)) when consuming <NUM>% of the supplied carbon monoxide was evaluated by repeating an operation of heating <NUM> of the obtained rhodium cluster-supporting catalyst (Rhcluster/BEA) for about <NUM> hours in an electric furnace at a heating rate of <NUM>/min to a peak heating temperature of <NUM> to <NUM> from room temperature while flowing an evaluation gas having the following composition over the catalyst, and cooling it to room temperature.

With respect to the temperature change in the above-described repetitive step of heating and cooling, as illustrated in <FIG>, the peak heating temperature becomes higher as it goes to the latter half, and a total of about <NUM> hours was spent.

In the repetitive step of heating and cooling, the evaluation was performed while raising the evaluation gas temperature to the peak heating temperature, i.e., in the temperature rising process. Similarly, in the repetitive step of heating and cooling, the evaluation was performed while lowering the evaluation gas temperature from the peak heating temperature, i.e., in the temperature dropping process.

For reference, with respect to a commercially available exhaust gas purification catalyst (Rh/Al<NUM>O<NUM>-CeO<NUM>-ZrO<NUM>) as Reference Comparative Example <NUM>, the evaluations in the temperature rising process and the temperature dropping process were performed in the same manner as in Reference Example <NUM>.

The evaluation results in the temperature rising process and the temperature dropping process are illustrated respectively in <FIG> as the difference between the result of Reference Example <NUM> and the result of Reference Comparative Example <NUM> (TCO(<NUM>%) of Reference Example <NUM>)-(TCO(<NUM>%) of Reference Comparative Example <NUM>). When the difference above takes a minus value, this indicates that TCO(<NUM>%) of Reference Example <NUM> is lower than TCO(<NUM>%) of Reference Comparative Example <NUM>, i.e., the low-temperature activity of the catalyst of Reference Example <NUM> is excellent. In <FIG>, the abscissa indicates the temperature (the peak temperature in <FIG>) at which an accelerated deterioration treatment was performed.

It is understood from <FIG> that the catalyst of Reference Example <NUM> provides an excellent exhaust gas purification performance relative to the catalyst of Reference Comparative Example <NUM> as the peak heating temperature becomes higher. This indicates that the catalyst of Reference Example <NUM> is less likely to deteriorate compared with the catalyst of Reference Comparative Example <NUM>.

Although not to be bound by theory, it is considered that in the catalyst of Reference Comparative Example <NUM>, rhodium of various sizes ranging from the monoatomic level to the submicrometer level was supported on the carrier to randomly cause sintering of rhodium particles by heat at the peak heating temperature and the catalyst was thereby deteriorated, whereas in the catalyst of Reference Example <NUM>, rhodium clusters were stably maintained within the pores of zeolite and in turn, the catalyst was not deteriorated due to heat at the peak heating temperature.

Incidentally, the change when the peak heating temperature is <NUM> and <NUM> is a change in the firing process of removing water molecules adsorbed to zeolite and therefore, the catalytic activity needs to be evaluated from the change substantially when the peak heating temperature is <NUM> or more.

In Reference Example <NUM> and Reference Comparative Example <NUM>, a catalyst in which rhodium clusters are supported on zeolite carrier particles or fumed silica carrier particles was obtained, and with respect to the obtained catalysts, the durability of the catalyst was evaluated.

In Reference Example <NUM>, rhodium clusters were supported on zeolite carrier particles in the same manner as in Reference Example <NUM> except that a rhodium target was used in place of the gold target and beta-type zeolite carrier particles (BEA) (Si/Al ratio: <NUM>) were used as the zeolite carrier particle.

With respect to the obtained rhodium cluster-supporting catalyst (Rhcluster/BEA), the temperature (TCO(<NUM>%)) when consuming <NUM>% of the supplied carbon monoxide was evaluated in the temperature rising process and in the temperature dropping process in the same manner as in Reference Example <NUM>.

In Reference Comparative Example <NUM>, rhodium clusters were supported on a fumed silica particle in the same manner as in Reference Example <NUM> except that a rhodium target was used in place of the gold target and a fumed silica particle was used as the carrier particle.

With respect to this rhodium cluster-supporting catalyst (Rhcluster/silica), the temperature (TCO(<NUM>%)) when consuming <NUM>% of the supplied carbon monoxide was evaluated in the temperature rising process and in the temperature dropping process in the same manner as in Reference Example <NUM>.

It is understood from <FIG> that the catalyst (Rhcluster/BEA) of Reference Example <NUM> in which rhodium clusters are supported on beta-type zeolite carrier particles (BEA) has a significantly excellent low-temperature activity at all peak heating temperatures, compared with the catalyst (Rhcluster/silica) of Reference Comparative Example <NUM> in which rhodium clusters are supported on a fumed silica carrier particle.

Although not to be bound by theory, this is considered to be attributable to the fact that since fumed silica used in the catalyst of Reference Comparative Example <NUM> does not have a pore, the rhodium clusters were supported only on the surface thereof and in the process of the rhodium being supported on the carrier and/or during the accelerated deterioration treatment, the rhodium cluster underwent aggregation or grain growth, i.e., the fact that while the rhodium cluster of the catalyst of Reference Example <NUM> is stably maintained within the pores of the zeolite carrier, the rhodium cluster of the catalyst of Reference Comparative Example <NUM> is present on the outer surface of the fumed silica carrier.

In Reference Example <NUM> and Reference Comparative Example <NUM>, a catalyst was obtained by supporting or not supporting a rhodium cluster on zeolite carrier particles, and the obtained catalysts were evaluated for the durability.

In Reference Example <NUM>, rhodium clusters were supported on zeolite carrier particles in the same manner as in Reference Example <NUM> except that a rhodium target was used in place of the gold target and ZSM-<NUM> zeolite carrier particles (MFI) (Si/Al ratio: <NUM>) were used as the zeolite carrier particle.

With respect to the obtained rhodium cluster-supporting catalyst (Rhcluster/MFI), the temperature (TCO(<NUM>%)) when consuming <NUM>% of the supplied carbon monoxide was evaluated in the temperature rising process and in the temperature dropping process in the same manner as in Reference Example <NUM>.

In Reference Comparative Example <NUM>, rhodium clusters were dispersed in acetone in the same manner as in Reference Example <NUM> except that a rhodium target was used in place of the gold target and carrier particles was not used. Thereafter, at the stage where the rhodium clusters were aggregated to form a rhodium cluster aggregate particles, ZSM-<NUM> zeolite carrier particles (MFI) (Si/Al ratio: <NUM>) was added as the zeolite carrier particles to the acetone to prepare the catalyst (Rhparticle/MFI) of Reference Comparative Example <NUM> in which the rhodium cluster aggregate particle was supported on the ZSM-<NUM> zeolite carrier particles (MFI).

With respect to this rhodium aggregate particle-supported catalyst (Rhparticle/MFI), the temperature (TCO(<NUM>%)) when consuming <NUM>% of the supplied carbon monoxide was evaluated in the temperature rising process and in the temperature dropping process in the same manner as in Reference Example <NUM>.

It is understood from <FIG> that the catalyst (Rhcluster/MFI) of Reference Example <NUM> in which rhodium clusters are supported on a ZSM-<NUM> zeolite carrier particles (MFI) has a significantly excellent low-temperature activity at all peak heating temperatures, compared with the catalyst (Rhparticle/MFI) of Reference Comparative Example <NUM> in which a rhodium cluster aggregate particles are supported on a ZSM-<NUM> zeolite carrier particles (MFI).

Although not to be bound by theory, this is considered to be attributable to the fact that since the particle diameter of the rhodium cluster used in the catalyst of Reference Example <NUM> is significantly smaller than the particle diameter of the rhodium cluster aggregate particle used in the catalyst of Reference Comparative Example <NUM>, a low-temperature catalytic activity specific to clusters was exhibited and a relatively large surface area could be provided for the catalytic reaction.

In Reference Examples <NUM> to <NUM>, a catalyst was obtained by supporting a rhodium cluster on zeolite carrier particles, and the obtained catalysts were evaluated for the durability.

Specifically, Reference Examples <NUM> to <NUM> were conducted as follows.

In Reference Examples <NUM> to <NUM>, rhodium clusters were supported on zeolite carrier particles in the same manner as in Reference Example <NUM> except that a rhodium target was used in place of the gold target and the following carrier particles were used as the zeolite carrier particle.

With respect to the obtained rhodium cluster-supporting catalysts of Reference Examples <NUM> to <NUM>, the temperature (TCO(<NUM>%)) when consuming <NUM>% of the supplied carbon monoxide was evaluated in the temperature rising process and in the temperature dropping process in the same manner as in Reference Example <NUM>.

The evaluation results in the temperature rising process and the temperature dropping process are illustrated respectively in <FIG> as the difference between the results of Reference Examples <NUM> to <NUM> and the result of Reference Comparative Example <NUM> (commercially available exhaust gas purification catalyst) (TCO(<NUM>%) of Reference Examples <NUM> to <NUM>)-(TCO(<NUM>%) of Reference Comparative Example <NUM>). When the difference above takes a minus value, this indicates that TCO(<NUM>%) of Reference Examples <NUM> to <NUM> is lower than TCO(<NUM>%) of Reference Comparative Example <NUM>, i.e., the low-temperature activity of the catalysts of Reference Examples <NUM> to <NUM> is excellent. In <FIG>, the abscissa indicates the temperature (the peak temperature in <FIG>) at which an accelerated deterioration treatment was performed.

It is understood from <FIG> that the catalysts of Reference Examples <NUM> to <NUM> provides an excellent or equivalent exhaust gas purification performance relative to the catalyst of Reference Comparative Example <NUM> as the peak heating temperature becomes higher. This indicates that the catalysts of Reference Examples <NUM> to <NUM> are less likely to deteriorate compared with the catalyst of Reference Comparative Example <NUM>.

Although not to be bound by theory, this is considered to be attributable to the fact that in the catalyst of Reference Comparative Example <NUM>, rhodium of various sizes ranging from the monoatomic level to the submicrometer level was supported on the carrier to randomly cause sintering of rhodium particles by heat at the peak heating temperature and the catalyst was thereby deteriorated, whereas in the catalysts of Reference Examples <NUM> to <NUM>, rhodium clusters were stably maintained within the pores of zeolite and in turn, the catalyst was not deteriorated due to heat at the peak heating temperature.

Incidentally, the catalyst of Reference Example <NUM> showed a poor catalyst performance in the temperature range of up to <NUM> compared with the catalyst of Reference Comparative Example <NUM>, but it is clearly understood from the curves of <FIG> that when the accelerated deterioration treatment is further continued, the performance of the catalyst of Reference Example <NUM> surpasses the performance of the catalyst of Reference Comparative.

Reviewing Reference Examples <NUM> and <NUM> using a ZSM-<NUM> zeolite carrier particles (MFI) as the carrier, the catalyst of Reference Example <NUM> using ZSM-<NUM> zeolite carrier particles (MFI(<NUM>)) having an Si/Al ratio of <NUM> exhibited good catalyst performance, compared with the catalyst of Reference Example <NUM> using ZSM-<NUM> zeolite carrier particles (MFI(<NUM>)) having an Si/Al ratio of <NUM>,<NUM>. This is considered to occur because the ZSM-<NUM> zeolite carrier particles (MFI(<NUM>)) having an Si/Al ratio of <NUM> has a larger number of acid sites than the ZSM-<NUM> zeolite carrier particles (MFI(<NUM>)) having an Si/Al ratio of <NUM>,<NUM> and supporting of the rhodium cluster on the zeolite carrier particles was successfully performed by an electrostatic action.

Reviewing Reference Examples <NUM> and <NUM> using a beta-type zeolite as the carrier, similarly to the case of MFI zeolite, the catalyst of Reference Example <NUM> using a beta-type zeolite carrier particles (BEA(<NUM>)) having an Si/Al ratio of <NUM>, i.e., a zeolite carrier particles having a relatively large number of acid sites exhibited good catalyst performance, compared with the catalyst of Reference Example <NUM> using beta-type zeolite carrier particles (BEA(<NUM>)) having an Si/Al ratio of <NUM>,<NUM>, i.e., zeolite carrier particles having a relatively small number of acid sites.

However, in the case of using beta-type zeolite carrier particles (BEA), the difference in the catalyst performance due to the different in the Si/Al ratio was not so large as in the case of using ZSM-<NUM> zeolite carrier particles (MFI). This is considered to occur because the beta-type zeolite carrier particle (BEA) substantially has a large surface solid acid strength and the effect of the difference in the amount of acid sites was less likely to develop.

For reference, the zeta potential (indicator of solid acid strength) of the zeolite carrier particles used in Reference Examples <NUM> to <NUM> is as follows:.

That is, it is considered that in the case of a beta-type zeolite carrier particle (BEA), despite a large Si/Al ratio, the zeolite potential was low and supporting of the rhodium cluster on the zeolite carrier particle was thereby successfully performed by an electrostatic action.

In order to confirm this understanding, a rhodium particle was supported on MFI(<NUM>) (zeta potential: -<NUM> mV) and on MFI(<NUM>) (zeta potential: -<NUM> mV) by laser ablation in liquid, as a result, coloring of the carrier caused by the supporting of rhodium particle on the carrier particle occurred more prominently in MFI(<NUM>) than in MFI(<NUM>).

It is understood from this result that in the case of MFI(<NUM>) having a relatively small zeta potential, i.e., relatively large acid strength, the rhodium particle was supported on the carrier particle in a relatively successful manner through an electrostatic interaction between the rhodium particle and acid sites of the carrier particle.

In Reference Examples <NUM> and <NUM>, a supported copper-cluster catalyst was obtained by a method of reduction in liquid, and the obtained catalysts were evaluated by means of fluorescence.

In Reference Example <NUM>, a zeolite carrier particle was dispersed in <NUM>-propanol to produce a zeolite carrier particle dispersion liquid, and copper(II) chloride as a copper ion source and sodium borohydride (NaBH<NUM>) as a reducing agent were mixed with the dispersion liquid to synthesize a copper cluster in the dispersion liquid. The thus-synthesized copper cluster had a positive charge and was thereby electrically drawn to acid sites of the zeolite carrier particle and supported thereon.

Specifically, copper(II) chloride and sodium borohydride were mixed using the apparatus illustrated in <FIG>.

More specifically, a water bath <NUM> at about <NUM> was disposed on a magnetic stirrer <NUM>, a flask <NUM> was disposed thereon, a dropping funnel <NUM> was disposed on the flask <NUM>, and the contents 84a of the dropping funnel <NUM> were added dropwise to the contents 83a of the flask <NUM> with stirring by means of a stirring bar 81a. The dropwise addition was performed for <NUM> hour while keeping the temperature by the water bath, and after the completion of dropwise addition, stirring was further performed for <NUM> hour while keeping the temperature by the water bath. Thereafter, stirring was further performed at room temperature for <NUM> hours, and the contents of the flask were then filtered and fired at a temperature of <NUM> for <NUM> hours in the atmosphere to obtain the supported copper-cluster catalyst of Reference Example <NUM>.

The contents 84a of the dropping funnel <NUM> and the contents 83a of the flask <NUM> in Reference Example <NUM> are shown together in Table <NUM> below.

The supported copper-cluster catalyst of Reference Example <NUM> was obtained in the same manner as in Reference Example <NUM> except that the contents 84a of the dropping funnel <NUM> and the contents 83a of the flask <NUM> were changed as shown in Table <NUM> below.

The supported copper-cluster catalysts prepared in Reference Examples <NUM> and <NUM> and as reference samples, a copper ion-exchanged zeolite carrier particle and a proton-type zeolite carrier particle were measured for the fluorescence spectrum at an excitation wavelength of <NUM>. The results are illustrated in <FIG>.

In <FIG>, the result as to Reference Example <NUM> is indicated by the spectrum (i), the result as to Reference Example <NUM> is indicated by the spectrum (ii), the result as to the copper ion-exchanged zeolite carrier particle as a reference sample is indicated by the spectrum (iii), and the result as to the proton-type zeolite carrier particle as a reference sample is indicated by the spectrum (iv).

As understood from <FIG>, the supported copper-cluster catalysts obtained in Reference Examples <NUM> and <NUM>, particularly in Reference Example <NUM>, showed a peak at about <NUM>. This peak is considered to be derived from the copper clusters. In the supported copper-cluster catalyst obtained in Reference Example <NUM>, the peak above is as broad as having a half width of about <NUM> and is considered to be derived from the copper clusters.

In addition, the supported copper-cluster catalyst obtained in Reference Example <NUM> was measured for the fluorescence spectrum at an excitation wavelength of <NUM> and the excitation spectrum at a fluorescence monitor wavelength of <NUM> and <NUM>. The results are illustrated in <FIG>.

In <FIG>, the result as to the excitation wavelength of <NUM> is indicated by the fluorescence spectrum (i), the result as to the fluorescence monitor wavelength of <NUM> is indicated by the spectrum (ii), and the result as to the fluorescence monitor wavelength of <NUM> is indicated by the spectrum (iii).

In <FIG>, fluorescence specific to the copper clusters are observed, and it is therefore understood that the copper clusters are supported on the carrier particle.

In Reference Example <NUM> and Reference Comparative Example <NUM>, rhodium cluster-supporting catalysts were obtained by using a method of laser ablation in liquid and an ion exchange-reduction method, respectively.

In Reference Example <NUM>, rhodium clusters were supported on a zeolite carrier particle by a method of laser ablation in liquid in the same manner as in Reference Example <NUM> except that a rhodium target was used in place of the gold target and ZSM-<NUM> zeolite carrier particles (Si/Al ratio: <NUM>) were used as the zeolite carrier particle.

In Reference Comparative Example <NUM>, a rhodium ion was supported on ZSM-<NUM> zeolite carrier particles (Si/Al ratio: <NUM>) by ion exchange, and then a metallic rhodium particle was supported on the zeolite carrier particle by reducing the rhodium ion, whereby rhodium clusters were supported on the zeolite carrier particle (ion exchange-reduction method). Rh(NO<NUM>)<NUM> was used as a rhodium ion source and NaBH<NUM> was used as a reducing agent.

The supported catalysts of Reference Example <NUM> and Reference Comparative Example <NUM> were measured for the fluorescence spectrum (excitation wavelength: <NUM>). The evaluation results of fluorescence spectrum normalized to the intensity per <NUM> of rhodium are illustrated in <FIG>.

It is understood from <FIG> that compared with Reference Comparative Example <NUM> using an ion exchange-reduction method, in Reference Example <NUM> using laser ablation in liquid, the fluorescence peak is large, i.e., a relatively large number of rhodium particles are supported in the cluster state on the zeolite carrier particle.

In Reference Example <NUM> and Reference Comparative Example <NUM>, supported gold-cluster catalysts were obtained by using a method of laser ablation in liquid and an ion exchange-reduction method, respectively.

In Reference Example <NUM>, gold clusters were supported on a zeolite carrier particle by a method of laser ablation in liquid in the same manner as in Reference Example <NUM> except that ZSM-<NUM> zeolite carrier particles (MFI) (Si/Al ratio: <NUM>,<NUM>) were used as the zeolite carrier particle.

In Reference Comparative Example <NUM>, a gold ion was supported on ZSM-<NUM> zeolite carrier particles (MFI) (Si/Al ratio: <NUM>,<NUM>) by ion exchange, and then gold clusters were supported on the zeolite carrier particle by reducing the gold ion (ion exchange-reduction method). Chloroauric acid (HAuCl<NUM>) was used as a gold ion source and NaBH<NUM> was used as a reducing agent.

With respect to the supported catalysts of Reference Example <NUM> and Reference Comparative Example <NUM>, the elemental composition of the supported catalyst as a whole was evaluated using inductively coupled plasma spectrometer (ICP-OES apparatus) (Agilent <NUM> manufactured by Agilent Technologies, Inc. and SPS3000 manufactured by Hitachi High-Tech Science Corporation).

With respect to the supported catalysts of Reference Example <NUM> and Reference Comparative Example <NUM>, the elemental composition of the supported catalyst surface was evaluated using a transmission electron microscope-energy dispersive X-ray spectroscopy (TEM-EDX) (JEM-2100F and JED-<NUM>, manufactured by JEOL Ltd.

It is understood from Table <NUM> that in the supported catalyst of Reference Example <NUM> obtained by the method of laser ablation in liquid, compared with the supported catalyst of Reference Comparative Example <NUM> obtained by the ion exchange-reduction method, the ratio of the proportion of gold element in the whole to the proportion of gold element in the surface is small, i.e., the gold clusters are relatively uniformly dispersed in the supported catalyst.

In Reference Example <NUM>, rhodium clusters were supported on a zeolite carrier particle by a method of laser ablation in liquid in the same manner as in Reference Example <NUM> except that a rhodium target was used in place of the gold target and ZSM-<NUM> zeolite carrier particles (Si/Al ratio: <NUM>) were used as the zeolite carrier particle. The amount of rhodium supported was <NUM> mass% relative to the zeolite carrier particle.

In Reference Comparative Example <NUM>, a rhodium ion was supported on ZSM-<NUM> zeolite carrier particles (Si/Al ratio: <NUM>) by ion exchange, and then a metallic rhodium particle was supported on the zeolite carrier particle by reducing the rhodium ion, whereby rhodium clusters were supported on the zeolite carrier particle (ion exchange-reduction method). Rh(NO<NUM>)<NUM> was used as a rhodium ion source and NaBH<NUM> was used as a reducing agent. The amount of rhodium supported was <NUM> mass% relative to the zeolite carrier particle.

With respect to the supported catalysts of Reference Example <NUM> and Reference Comparative Example <NUM>, a pretreatment was performed by adsorbing oxygen to the supported catalyst at <NUM> for <NUM> hour in a <NUM> vol% oxygen atmosphere and removing excess oxygen at <NUM> for <NUM> hour in a helium atmosphere.

With respect to the supported catalysts above subjected to the pretreatment, a test by hydrogen temperature-programmed reduction method (H<NUM>-TPR) was performed by flowing a reducing gas containing <NUM> vol% of hydrogen and the balance helium at a spatial velocity of <NUM>,<NUM>-<NUM> while raising the temperature at a rate of <NUM>/min from <NUM>.

The result as to the supported catalyst of Reference Example <NUM> is illustrated in <FIG>, and the result as to the supported catalyst of Reference Comparative Example <NUM> is illustrated in <FIG>. The peak/noise ratio of the peak indicated by an arrow of <FIG> was <NUM> (noise level: <NUM>%), and the peak/noise ratio of the peak indicated by an arrow of <FIG> was <NUM> (noise level: <NUM>%).

It is understood from these graphs that both of the supported catalysts of Reference Example <NUM> and Reference Comparative Example <NUM> have a relatively large peak of reaction between hydrogen supplied and oxygen adsorbed to the cluster-supporting catalyst, i.e., a peak with a peak/noise ratio of <NUM> or more, in the temperature range of <NUM> or less, i.e., have low-temperature activity.

With respect to the supported catalysts of Reference Example <NUM> and Reference Comparative Example <NUM>, a thermal endurance treatment was performed by subjecting the supported catalyst to heating for <NUM> hours in an atmosphere at <NUM> containing <NUM> vol% of oxygen and the balance helium and then to heating for <NUM> hour in an atmosphere at <NUM> containing <NUM> vol% of hydrogen and the balance helium.

With respect to the supported catalysts above subjected to the thermal endurance treatment, a pretreatment was performed as described above.

With respect to the supported catalysts above subjected to the pretreatment, a H<NUM>-TPR test was performed as described above.

It is understood from <FIG> that the supported catalyst of Reference Example <NUM> has a relatively large reaction peak in the temperature range of <NUM> or less, i.e., has low-temperature activity. In addition, it is understood from <FIG> that the supported catalyst of Reference Comparative Example <NUM> does not have a substantial peak in the temperature range of <NUM> or less, i.e., does not have a peak with a peak/noise ratio of <NUM> or more. In this way, the supported catalyst of Reference Comparative Example <NUM> does not have a substantial peak in the temperature range of <NUM> or less, and this means that the supported catalyst does not have low-temperature activity. That is, it is understood that in the supported catalyst of Reference Comparative Example <NUM> obtained by the ion exchange-reduction method, the dispersibility of the cluster particle was low and in turn, the heat resistance was poor.

In Reference Example <NUM> and Reference Comparative Example <NUM>, palladium cluster-supporting catalysts were obtained by using a method of laser ablation in liquid and an ion exchange-reduction method, respectively.

In Reference Example <NUM>, palladium clusters were supported on a zeolite carrier particle by a method of laser ablation in liquid in the same manner as in Reference Example <NUM> except that a palladium target was used in place of the gold target and ZSM-<NUM> zeolite carrier particles (Si/Al ratio: <NUM>) were used as the zeolite carrier particle. The amount of palladium supported was <NUM> mass% relative to the zeolite carrier particle.

In Reference Comparative Example <NUM>, a palladium ion was supported on ZSM-<NUM> zeolite carrier particles (Si/Al ratio: <NUM>) by ion exchange, and then a metallic palladium particle was supported on the zeolite carrier particle by reducing the palladium ion, whereby palladium clusters were supported on the zeolite carrier particle (ion exchange-reduction method). Pd(NH<NUM>)<NUM>Cl<NUM>·H<NUM>O (tetraamminepalladium(II) chloride monohydrate) was used as a palladium ion source and NaBH<NUM> was used as a reducing agent. The amount of palladium supported was <NUM> mass% relative to the zeolite carrier particle.

A model gas containing <NUM> vol% of carbon monoxide, <NUM> vol% of oxygen and the balance helium was flowed at a spatial velocity of <NUM>,<NUM>-<NUM> over the supported catalysts subjected to the thermal endurance treatment, and the number of molecules of the carbon monoxide molecule capable of being oxidized to carbon dioxide molecule by one palladium atom at a temperature of <NUM> in the temperature dropping process was evaluated by performing a temperature rising process of raising the temperature at a rate of <NUM>/min to <NUM> from room temperature and then performing a temperature dropping process of lowering the temperature to room temperature.

The number of molecules can be obtained by dividing the molar number of the carbon dioxide molecule in the model gas flowing per second after reaction by the molar number of palladium as a catalyst metal in the supported catalyst.

The results as to the supported catalysts of Reference Example <NUM> and Reference Comparative Example <NUM> are illustrated in <FIG>. It is seen from <FIG> that in the supported catalyst of Reference Example <NUM> obtained by the laser ablation method, the number of molecules of the carbon monoxide molecule capable of being oxidized to carbon dioxide molecule by one palladium atom was near <NUM> and on the other hand, in the supported catalyst of Reference Comparative Example <NUM> obtained by the ion exchange-reduction method, the number did not reach <NUM>. This indicates that in the supported catalyst of Reference Comparative Example <NUM> obtained by the ion exchange-reduction method, the dispersibility of the cluster particle was low and in turn, the heat resistance was poor.

In Reference Example <NUM> and Reference Comparative Example <NUM>, platinum cluster-supporting catalysts were obtained by using a method of laser ablation in liquid and an ion exchange-reduction method, respectively.

In Reference Example <NUM>, platinum clusters were supported on a zeolite carrier particle by a method of laser ablation in liquid in the same manner as in Reference Example <NUM> except that a platinum target was used in place of the gold target and ZSM-<NUM> zeolite carrier particles (Si/Al ratio: <NUM>) were used as the zeolite carrier particle. The amount of platinum supported was <NUM> mass% relative to the zeolite carrier particle.

In Reference Comparative Example <NUM>, a platinum ion was supported on ZSM-<NUM> zeolite carrier particles (Si/Al ratio: <NUM>) by ion exchange, and then a metallic platinum particle was supported on the zeolite carrier particle by reducing the platinum ion, whereby platinum clusters were supported on the zeolite carrier particle (ion exchange-reduction method). Pt(NH<NUM>)<NUM>Cl<NUM>·xH<NUM>O (tetraammineplatinum(II) chloride monohydrate) was used as a platinum ion source and NaBH<NUM> was used as a reducing agent. The amount of platinum supported was <NUM> mass% relative to the zeolite carrier particle.

A model gas containing <NUM> vol% of carbon monoxide, <NUM> vol% of oxygen and the balance helium was flowed at a spatial velocity of <NUM>,<NUM>-<NUM> over the supported catalysts subjected to the thermal endurance treatment, and the number of molecules of the carbon monoxide molecule capable of being oxidized to carbon dioxide molecule by one platinum atom at a temperature of <NUM> in the temperature dropping process was evaluated by performing a temperature rising process of raising the temperature at a rate of <NUM>/min to <NUM> from room temperature and then performing a temperature dropping process of lowering the temperature to room temperature.

The results as to the supported catalysts of Reference Example <NUM> and Reference Comparative Example <NUM> are illustrated in <FIG>. It is seen from <FIG> that in the supported catalyst of Reference Example <NUM> obtained by the laser ablation method, the number of molecules of the carbon monoxide molecule capable of being oxidized to carbon dioxide molecule by one platinum atom was near <NUM> and on the other hand, in the supported catalyst of Reference Comparative Example <NUM> obtained by the ion exchange-reduction method, the number did not reach <NUM>. This indicates that in the supported catalyst of Reference Comparative Example <NUM> obtained by the ion exchange-reduction method, the dispersibility of the cluster particle was low and in turn, the heat resistance was poor.

In Reference Example <NUM> and Reference Comparative Example <NUM>, supported copper-cluster catalysts were obtained by using a method of laser ablation in liquid and an ion exchange-reduction method, respectively.

In Reference Example <NUM>, copper clusters were supported on a zeolite carrier particle by a method of laser ablation in liquid in the same manner as in Reference Example <NUM> except that a copper target was used in place of the gold target and a chavazite (CHA)-type zeolite carrier particle were used as the zeolite carrier particle. The amount of copper supported was <NUM> mass% relative to the zeolite carrier particle.

In Reference Comparative Example <NUM>, a copper ion was supported on a chavazite (CHA)-type zeolite carrier particle by ion exchange, and then a metallic copper particle was supported on the zeolite carrier particle by reducing the copper ion, whereby copper clusters were supported on the zeolite carrier particle (ion exchange-reduction method). Copper nitrate was used as a copper ion source and NaBH<NUM> was used as a reducing agent. The amount of copper supported was <NUM> mass% relative to the zeolite carrier particle.

With respect to the supported catalysts of Reference Example <NUM> and Reference Comparative Example <NUM>, nitric oxide was adsorbed to the supported catalyst through heating at <NUM> for <NUM> hour in an atmosphere containing <NUM> vol% of oxygen and the balance helium, heating at <NUM> for <NUM> minutes in an atmosphere containing <NUM> vol% of helium, lowering of the ambient temperature to <NUM>, holding for <NUM> hour in an atmosphere containing <NUM> ppm by volume of nitric oxide and the balance helium, and holding for <NUM> hour in an atmosphere containing <NUM> vol% of helium.

The supported catalyst having adsorbed thereto nitric oxide was heated at a temperature rise rate of <NUM>/min to <NUM> in an atmosphere containing <NUM> vol% of helium, and the amount of nitric oxide desorbed during the heating was detected by a mass spectrometer to obtain a nitric oxide temperature-programmed desorption spectrum. Incidentally, the gas flow rate in the atmosphere was <NUM> sccm in all cases.

The results as to the supported catalysts of Reference Example <NUM> and Reference Comparative Example <NUM> are illustrated in <FIG>, respectively.

It is revealed from <FIG> that in the supported catalyst of Reference Example <NUM> obtained by the laser ablation method, the maximum peak in the range of <NUM> to <NUM> is present at about <NUM>. On the other hand, it is revealed from <FIG> that in the supported catalyst of Reference Comparative Example <NUM> obtained by the ion exchange-reduction method, the maximum peak in the range of <NUM> to <NUM> is present at about <NUM>. A sharp peak observed at a temperature of about <NUM> or less is thought of as a measurement error due to fluctuation of the measurement temperature.

The difference in the temperature between maximum peaks illustrated in <FIG> indicates that the supported catalyst of Reference Example <NUM> obtained by the laser ablation method and the supported catalyst of Reference Comparative Example <NUM> obtained by the ion exchange-reduction method have different structures from each other.

In Reference Example <NUM> and Reference Comparative Example <NUM>, platinum cluster-supporting catalysts were obtained by using an positive-negative inversion method and an ion exchange-reduction method, respectively.

In Reference Example <NUM>, zeolite MFI(<NUM>) was added to <NUM> of an aqueous solution containing <NUM> of H<NUM>[PtCl<NUM>] in pure water, a pulsed laser is converged and introduced into this aqueous solution to decompose H<NUM>[PtCl<NUM>] and produce a positively charged platinum cluster, and the positively charged platinum clusters were supported on the acid sites of zeolite through an electrostatic interaction.

In Reference Comparative Example <NUM>, H<NUM>[PtCl<NUM>] in pure water was supported on zeolite MFI(<NUM>) by ion exchange. The amount of platinum supported was <NUM> mass% relative to the zeolite carrier particle.

The platinum cluster-supporting catalysts of Reference Example <NUM> and Reference Comparative Example <NUM> were measured for the fluorescence spectrum (excitation wavelength: <NUM>). The evaluation results of fluorescence spectrum are illustrated in <FIG>. In <FIG>, the results as to Reference Example <NUM> is indicated by the spectrum (i), and the result as to Reference Comparative Example <NUM> is indicated by the spectrum (ii).

In <FIG>, the fluorescence signal at near <NUM> is a spectrum in which fluorescent emissions from the gold cluster of about tetramer were overlapped. Accordingly, <FIG> reveals that in the platinum cluster-supporting catalyst of Reference Example <NUM>, a relatively large amount of a platinum cluster around tetramer is supported on the carrier particle and on the other hand, in the supported-platinum catalyst of Reference Comparative Example <NUM>, such clusters are not present in a significant manner.

In Reference Comparative Example <NUM>, a rhodium ion was supported on ZSM-<NUM> zeolite carrier particles (Si/Al ratio: <NUM>) by ion exchange, and then rhodium clusters were supported on the zeolite carrier particle by reducing the rhodium ion. Rh(NO<NUM>)<NUM> was used as a rhodium ion source and NaBH<NUM> was used as a reducing agent. The amount of rhodium supported was <NUM> mass% relative to the zeolite carrier particle.

A thermal endurance treatment was performed by heating the supported catalyst for <NUM> hour in an atmosphere at <NUM> containing <NUM> vol% of oxygen, <NUM> vol% of carbon monoxide and the balance helium.

A model gas containing <NUM> vol% of <NUM>NO, <NUM> vol% of CO and the balance helium was flowed at a spatial velocity of <NUM>,<NUM>-<NUM> over the supported catalysts subjected to the thermal endurance treatment, and the nitric oxide reduction reaction was measured by raising the temperature at a rate of <NUM>/min to <NUM> from room temperature (temperature rising process) and then lowering the temperature to room temperature (temperature dropping process).

With respect to the concentration change of each component due to the nitric oxide reduction, the evaluation result as to the catalyst of Reference Example <NUM> is illustrated in <FIG>, and the evaluation result as to the catalyst of Reference Comparative Example <NUM> is illustrated in <FIG>.

In <FIG>, a peak of nitrogen oxide appears in the range of <NUM> to <NUM>, and this is a concentration rise due to desorption of nitric oxide adsorbed to the catalyst. When the reaction temperature further rises, the concentration of nitric oxide decreases, and a reaction of reducing nitric oxide (<NUM>NO) by carbon monoxide (CO) to produce nitrogen (N<NUM>) starts.

In the catalyst of Reference Example <NUM>, the reaction temperature at the time of half of the nitric oxide supplied being reduced into nitrogen, i.e., the reaction temperature at the time of the nitrogen concentration becoming <NUM> vol%, is about <NUM> in the temperature rising process and <NUM> in the temperature dropping process, whereas in the catalyst of Reference Comparative Example <NUM>, the reaction temperature was about <NUM> in the temperature rising process and <NUM> in the temperature dropping process. It is therefore revealed that the catalyst of Reference Example <NUM> obtained by the method of laser ablation in liquid has excellent low-temperature activity, compared with the catalyst of Reference Comparative Example <NUM> obtained by the ion exchange-reduction method.

The evaluation results of the number of molecules of the nitric oxide molecule capable of being reduced to nitrogen by one rhodium atom at a temperature of <NUM> in the temperature dropping process are illustrated in <FIG>.

<FIG> reveals that in the supported catalyst of Reference Example <NUM> obtained by the laser ablation method, the number of molecules of nitrogen monoxide molecule capable of being purified in <NUM> second by one rhodium atom exceeded <NUM> and on the other hand, in the catalyst of Reference Comparative Example <NUM> obtained by the ion exchange-reduction method, the number did not reach <NUM>. It is therefore apparent that the catalyst of Reference Example <NUM> obtained by the laser ablation method has excellent low-temperature activity, compared with the catalyst of Reference Comparative Example <NUM> obtained by the ion exchange-reduction method.

In Reference Examples <NUM> and <NUM>, a platinum cluster-supporting catalyst and a rhodium cluster-supporting catalyst were obtained respectively by using a method of laser ablation in liquid. In Reference Comparative Example <NUM>, a general three-way catalyst in which platinum, rhodium and palladium are supported on a mixed powder of alumina carrier particles and ceria-zirconia carrier particles was used.

In Reference Comparative Example <NUM>, a general three-way catalyst in which platinum, rhodium and palladium are supported on a mixed powder of alumina carrier particles and ceria-zirconia carrier particles was used. The amounts of platinum, rhodium and palladium supported were <NUM> mass%, <NUM> mass% and <NUM> mass%, respectively, relative to the carrier powder.

The catalysts of Reference Examples <NUM> and <NUM> and Reference Comparative Example <NUM> were held at <NUM> for <NUM> hour in an atmosphere containing <NUM> ppm by volume of carbon monoxide and the balance helium to adsorb carbon monoxide to the supported catalyst, and then an oxygen oxidation reaction test of adsorbed carbon monoxide was performed by heating the supported catalyst having adsorbed thereto carbon monoxide at a temperature rise rate of <NUM>/min to <NUM> in an atmosphere containing <NUM> vol% of oxygen and the balance helium. During these treatments, the spatial velocity was <NUM>,<NUM>-<NUM>.

In addition, the catalysts of Reference Examples <NUM> and <NUM> and Reference Comparative Example <NUM> were cleaned by performing the following treatments (i) to (iv):.

The catalysts of Reference Examples <NUM> and <NUM> and Reference Comparative Example <NUM> subjected to the cleaning treatment were subjected to the above-described oxygen oxidation reaction test of adsorbed carbon monoxide.

With respect to the catalysts of Reference Examples <NUM> and <NUM> and Reference Comparative Example <NUM>, the results of the oxygen oxidation reaction test of adsorbed carbon monoxide before and after the cleaning treatment are illustrated in <FIG>.

As apparent from <FIG>, in the catalysts of Reference Examples <NUM> and <NUM> obtained by the method of laser ablation in liquid, a signal on the low temperature side of <NUM> or less was present before and after the cleaning treatment, whereas in the catalyst of Reference Comparative Example <NUM> that is a general three-way catalyst, the difference in the evaluation result between before and after the cleaning treatment was large and a signal on the low temperature side of <NUM> or less was not observed.

Claim 1:
A cluster-supporting catalyst, comprising boron-substituted zeolite particles, and catalyst metal clusters supported within the pores of the boron-substituted zeolite particles, wherein
the metal is rhodium, and
the boron-substituted zeolite particles are silicalite type (code: MFI) having an Si/(Al and B) ratio of <NUM> or less,
wherein the cluster-supporting catalyst is obtainable by:
providing a dispersion liquid containing acetone and the boron-substituted zeolite particles dispersed in acetone, and
forming, in the dispersion liquid, catalyst metal clusters having a positive charge, and supporting the catalyst metal clusters on the acid sites within the pores of the boron-substituted zeolite particles through an electrostatic interaction,
wherein the clusters are formed in the dispersion liquid by a method of laser ablation in liquid, using a NAD:YAG laser with an intensity of 2W.