NOX purification catalyst

This is a Continuation of application Ser. No. 13/983,698 filed Aug. 5, 2013, which is the National Stage of PCT/JP2011/052974 filed Feb. 7, 2011, the entire disclosure of the prior applications, application Ser. No. 13/983,698 and PCT/JP2011/052974 are hereby incorporated by reference.

1. Field of the Invention

The present invention relates to a nitrogen oxide (and more, sometimes abbreviated as “NO x ”) purification catalyst, more particularly relates to a novel NO x purification catalyst comprised of particles including gold atoms and iron atoms and able to purify NO x at a low temperature and/or in an oxidizing atmosphere.

2. Description of the Related Art

In recent years, from the viewpoint of protection of the global environment, exhaust gas regulations have been toughened around the world with each passing year. As one means for dealing with this, in internal combustion engines, exhaust gas purification catalysts are being used. In such exhaust gas purification catalysts, in order to efficiently remove HC (hydrocarbons), carbon monoxide (CO), and NO x in the exhaust gas, platinum, gold, rhodium, and other precious metals have been used as catalyst ingredients.

In motor vehicles using such purification catalysts, for example gasoline engine vehicles and diesel engine vehicles, various systems are being used to improve the catalytic activity and the fuel economy. For example, for improving fuel economy during steady state operation fuel is burned under conditions of a lean (excess oxygen) air-fuel ratio (A/F), while to improve the catalytic activity, fuel is burned under temporarily stoichiometric (stoichiometric air-fuel ratio, A/F=14.7) to rich (excess fuel) conditions.

These conditions apply because conventionally known platinum, gold, rhodium, and other precious metal catalysts exhibit low NO x purification performances at low temperature and under oxidizing conditions. To improve the purification performance, the purification catalyst has to be operated high in temperature and HC or CO etc. has presently to be added to establish a reducing atmosphere. Therefore, even during steady state operation, it is not possible to increase the air-fuel ratio (A/F).

With the above precious metal catalysts, there is therefore a limit to the improvement of the fuel economy. In such conventionally known precious metal catalysts, to obtain purification performance, energy for raising the purification catalysts to a high temperature, fuel for temporarily exposing the purification catalysts to a reducing atmosphere, and reduction of the air-fuel ratio (A/F) in the engine are all required. To improve the fuel economy in automobile engines and other internal combustion engines, therefore, a new purification catalyst able to exhibit a good NO x purification performance at a low temperature and/or in an oxidizing atmosphere is being sought.

Furthermore, all of the above precious metal catalysts face the problem of resource depletion. NO x purification catalysts using other metals to obtain purification performances equal to or better than those of conventional precious metal catalysts or able to reduce the amounts of the precious metals used are therefore being sought.

For example, Japanese Patent Publication (A) No. 10-216518 describes a gold alloy catalyst comprised of gold and one or more metals (M) selected from platinum, palladium, silver, copper, and nickel, having a weight ratio Au/M of 1/9 to 9/1, and having an amount of solute gold in the alloy of 20 to 80 wt %. Further, the catalysts shown as specific examples in this publication are catalysts supporting a gold alloy of gold and a metal of palladium or platinum on an Al 2 O 3 carrier. These exhibit a high NO x purification performance in a reducing atmosphere, but have a low NO x purification performance at a low temperature and/or in an oxidizing atmosphere.

Further, Japanese Patent Publication (A) No. 10-216519 describes a metal particulate-carrying oxide catalyst produced by heat treating a metal compound composed of one or more elements selected from gold, platinum, palladium, rhodium, silver, iridium, ruthenium, and osmium and one or more elements selected from metals of strontium, niobium, lithium, lanthanum, aluminum, silicon, magnesium, calcium, barium, cerium, neodymium, titanium, iron, cobalt, nickel, copper, zinc, zirconium, vanadium, tantalum, chromium, molybdenum, tungsten, sodium, potassium, beryllium, scandium, yttrium, indium, tin, lead, and bismuth in an atmosphere containing oxygen. Further, the catalysts shown as specific examples in this publication are catalysts composed of gold alloy incorporating strontium or lanthanum on an Al 2 O 3 carrier. They exhibit a relatively high NO x purification performance in a reducing atmosphere, but have an extremely low NO x purification performance at a low temperature and/or in an oxidizing atmosphere.

Further, Japanese Patent Publication (A) No. 2001-239161 describes a low temperature harmful gas-purification catalyst comprised of a metal oxide on carbonaceous material carrier using high temperature/high pressure fluid to carry ultrafine particles of at least one type of metal selected from the group comprising platinum, palladium, rhodium, ruthenium, iridium, osmium, gold, silver, copper, manganese, iron, and nickel. Further, the catalysts shown as specific examples in this publication are purification catalysts supporting one of type of platinum, palladium, rhodium, ruthenium, nickel, or gold. They exhibit a good NO x purification performance in a reducing atmosphere.

Furthermore, Japanese Patent Publication (A) No. 2003-190787 describes an engine exhaust gas purification catalyst mainly comprised of 12CaO.7Al 2 O 3 and carrying one or more elements selected from gold, silver, iron, zinc, manganese, cerium, and platinum group elements. Further, this publication shows, as specific examples, that purification catalysts mainly comprised of 12CaO.7Al 2 O 3 carrying one element out of gold, silver, platinum, palladium, copper, iron, zinc, manganese, cerium, or rhodium or two elements out of silver and rhodium, ruthenium, or copper fired at 600° C. exhibit the effect of reducing the combustion temperature by the oxidation reaction of the particulate matter (PM) by oxygen radicals. However, this publication does not prescribe the positional relationship of the two types of metals. It is unclear if the catalysts shown as specific examples in this publication exhibit an NO x purification performance.

As related art, therefore, there are the above Japanese Patent Publication (A) No. 10-216518, Japanese Patent Publication (A) No. 10-216519, Japanese Patent Publication (A) No. 2001-239161, and Japanese Patent Publication (A) No. 2003-190787.

Therefore, in these known purification catalysts, it was difficult to reduce the amounts of precious metals used and obtain good NO x purification performance at a low temperature and/or in an oxidizing atmosphere.

Therefore, an object of the present invention is to provide a catalyst able to reduce the amount of use of precious metal whilst exhibiting a good NO x purification performance at a low temperature and/or in an oxidizing atmosphere.

The inventors engaged in intensive research for the purpose of realizing the above object and as a result have completed the present invention.

The present invention provides a nitrogen oxide purification catalyst comprised of particles having an average particle size of 0.2 to 100 nm and including gold atoms and iron atoms in a state of close proximity. The “average particle size” in the present invention is the average of the particle sizes measured using a method explained in detail later in the section on examples.

According to the present invention, it is possible to obtain a catalyst that uses less precious metal and exhibits a good NO x purification performance at a low temperature and/or in an oxidizing atmosphere.

These and other objects and features of the present invention will become clearer from the following description of the preferred embodiments given with reference to the attached drawings, wherein:

FIG. 1A is a model of a nanoparticle in an NO x purification catalyst of an embodiment of the present invention;

FIG. 1B is a schematic view of an NO x purification catalyst of an embodiment of the present invention;

FIG. 2A is a graph showing the relationship between the iron concentration and particle size in the model shown in FIG. 1A ;

FIG. 2B is a graph enlarging part of FIG. 2A ;

FIG. 3A is a TEM image of nanoparticles of AuFe (Au:Fe=50:50, atomic ratio, same below) of an AuFe/Al 2 O 3 -based NO x purification catalyst obtained in an example of the invention;

FIG. 3B is a TEM image changing the scale of FIG. 3A ;

FIG. 4A is a TEM image of nanoparticles of AuFe (Au:Fe=50:50) of an AuFe/SiO 2 -based NO x purification catalyst obtained in another example of the invention;

FIG. 4B is a TEM image changing the scale of FIG. 4A ;

FIG. 4C is a TEM image of nanoparticles of AuFe (Au:Fe=50:50) of an AuFe/SiO 2 -based NO x purification catalyst obtained in another example of the invention;

FIG. 4D is a TEM image changing the scale of FIG. 4C ;

FIG. 5A is a TEM image of nanoparticles of AuFe (Au:Fe=25:75) of an AuFe/SiO 2 -based NO x purification catalyst obtained in another example of the invention;

FIG. 5B is a TEM image changing the scale of FIG. 5A ;

FIG. 6 is a histogram showing the particle size distribution of 100 nanoparticles in an HRTEM image obtained for nanoparticles of AuFe (Au:Fe=50:50) of an AuFe/Al 2 O 3 -based NO x purification catalyst obtained in an example of the invention;

FIG. 7 is a distribution histogram of the particle size showing the particle size distribution of 100 nanoparticles in an HRTEM image obtained for nanoparticles of AuFe (Au:Fe=50:50) of an AuFe/SiO 2 -based NO x purification catalyst obtained in another example of the invention;

FIG. 8 is a TEM-EDS spectrum of AuFe (Au:Fe=50:50) nanoparticles of an AuFe/Al 2 O 3 -based NO x purification catalyst obtained in an example of the invention;

FIG. 9 is a TEM-EDS spectrum of AuFe (Au:Fe=50:50) nanoparticles of an AuFe/SiO 2 -based NO x purification catalyst obtained in another example of the invention;

FIG. 10 is a TEM-EDS spectrum of AuFe (Au:Fe=25:75) nanoparticles of an AuFe/SiO 2 -based NO x purification catalyst obtained in another example of the invention;

FIG. 11 is an X-ray diffraction spectrum of AuFe (Au:Fe=25:75) nanoparticles;

FIG. 12 is a graph comparing the NO—CO catalytic activity of an AuFe (Au:Fe=50:50)/Al 2 O 3 -based NO x purification catalyst obtained in an example of the invention and NO x purification catalysts outside the range of the present invention;

FIG. 13 is a graph comparing the NO—H 2 catalytic activity of an AuFe (Au:Fe=50:50)/SiO 2 -based NO x purification catalyst obtained in another example of the invention and nanometallic NO x purification catalysts outside the range of the present invention;

FIG. 14 is a graph showing the NO—H 2 catalytic activity of an AuFe (Au:Fe=25:75)/SiO 2 -based NO x purification catalyst obtained in another example of the invention; and

FIG. 15 is a phase diagram (calculated) of Au—Fe copied from Binary Alloy Phase Diagram, vol. 2, p. 259, 1984.

The NO x purification catalyst of the present invention has to include gold atoms and iron atoms in a state of close proximity in particles.

The expression “gold atoms and iron atoms in a state of close proximity in particles” means a state in which at least one of the gold atoms is contiguous with at least one of the iron atoms in primary particles that are nanoparticles.

Below, referring to the drawings, embodiments of the present invention will be explained in detail.

The NO x purification catalyst of the present invention, as shown in FIG. 1A , envisions, an ideal spherical particle cut between out from nearby atoms of a cut out of a face-centered cubic lattice (cuboctahedron structure). As shown in FIG. 1B , the catalysts can be understood as a support on which is supported Au—Fe bimetal nanoparticulates when assuming at least one gold atom covered by iron atoms at the surface first layer and second layer carrier side half surfaces. In this model, the iron atoms at the surfacemost part of the particulate are set to be in close proximity to the gold atoms. Further, the model shown in FIG. 1B assumes iron covers the surface first layer and second layer of carrier side surface by virtue of presence of the oxidizing gas and the interaction with the carrier oxygen. We justify this assumption on the basis that iron has a larger bonding force with oxygen compared with gold in an oxidizing atmosphere. Iron therefore more easily appears at the nanoparticle and also easily bonds with the carrier, which also contains oxygen.

According to this model, it is understood that the NO x purification catalyst that constitutes the present invention, as shown in FIG. 2A , can exist as metal nanoparticles having a particle size of 0.2 to 30 nm with a range of concentration of Au:Fe=7:93 to 91:9 (at %) and, from FIG. 2B , can have metal nanoparticles of a range of concentrations of Au:Fe=8:92 to 26:74 (at %) and, in this matter, a mean particle size of for example about 0.9 to 2.5 nm.

Referring to FIG. 3A and FIG. 3B , the AuFe (Au:Fe=50:50)/Al 2 O 3 -based NO x purification catalyst of the examples discussed is composed of nanoparticles of AuFe particles having a particle size of 4.97±0.75 nm.

Referring to FIG. 6 , this AuFe (Au:Fe=50:50)/Al 2 O 3 -based NO x purification catalyst has a particle size distribution, found for 100 particles in an HRTEM image, of 3 to 6.5 nm in range.

Referring to FIG. 8 , it is shown that the particles of the NO x purification catalyst (Au:Fe=50:50) include both the ingredients of gold and iron from the TEM-EDS spectrum.

Further, from FIG. 12 , it can be seen that among NO x purification catalysts having Al 2 O 3 as a carrier, the AuFe-based NO x purification catalysts that constitute the present invention all exhibit distinctive NO—CO catalytic activities as compared to Au-, AuW-, AuRe-, and AuMn-based NO x purification catalysts—all of which had NO conversion rates at 500° C. of 0%.

Referring to FIG. 4A , FIG. 4B , FIG. 4C , and FIG. 4D , the AuFe (Au:Fe=50:50)/SiO 2 -based NO x purification catalysts that represent examples of the present invention are composed of nanoparticles of AuFe with a mean particle size of 3.94±0.56 nm or 4.72±0.77 nm.

Referring to FIG. 7 , it is shown that the particle size distribution found for 100 particles in multiple HRTEM images of with AuFe (Au:Fe=50:50)/Al 2 O 3 -based NO x purification catalyst was 3 to 7.5 nm in range.

Referring to FIG. 9 , it is shown that the NO x purification catalyst particles, according to the TEM-EDS spectrum, include both ingredients of gold and iron.

Further, from FIG. 13 , among the catalysts having SiO 2 as carriers, the AuFe (Au:Fe=50:50)/SiO 2 -based NO x purification catalysts obtained in the examples of the invention have a higher NO—H 2 catalytic activity than NO x purification catalysts comprised of SiO 2 carriers on which iron alone or gold alone is carried, which are outside the scope of the present invention, in the 200 to 400° C. range.

Referring to FIG. 5A and FIG. 5B , the AuFe (Au:Fe=25:75)/SiO 2 -based NO x purification catalysts of the example are comprised of nanoparticles of AuFe of a mean particle size of 3.86±0.96 nm.

Referring to FIG. 10 , the NO x purification catalyst particles include both the ingredients of gold and iron from the TEM-EDS spectrum.

Referring to FIG. 11 , the Fe(—Au) peak in the X-ray diffraction spectrum is broad. This shows that the Au—Fe particulate is amorphous.

Further, from FIG. 14 , the AuFe (Au:Fe=25:75)/SiO 2 -based NO x purification catalyst reported as an example of the invention has a high NO—H 2 catalytic activity in the 200 to 400° C. range.

The NO x purification catalyst of the present invention, as explained above, is comprised of particles having an average particle size of 0.2 to 100 nm and including gold atoms and iron atoms in a state of close proximity. As shown in FIG. 15 , the phase diagram for Au—Fe reveals that gold and iron cannot form a solid solution in the bulk or thin films for Au:Fe ratios of 8:92 to 26:74 (at %). That is, it is considered possible to form bimetallic nanoparticles with a range of compositions which cannot form a bimetal, to achieve compositions of Au:Fe in the range 7:93 to 91:9 (at %) and thereby obtain good NO x purification catalyst activity.

The NO x purification catalyst of the present invention has to have gold atoms and iron atoms present in a state of close proximity in nanoparticulates. For this reason, the region where the atoms are in close proximity may include other metal atoms able to alloy with the two atoms. Therefore, when it is necessary to use a carrier, the NO x purification catalyst of the present invention can be obtained by, for example, using nanoparticles of the material forming the carrier as cores for obtaining nanoparticles in which the two metals are in close proximity (that is, are alloyed).

As other metal atoms able to alloy with both the gold atoms and iron atoms, for example, tungsten (W), which can improve the heat resistance of Au by alloying, may be suggested.

Further, as the carrier, Al 2 O 3 , SiO 2 , CeO 2 , CeO 2 —ZrO 2 , and other metal oxides or carbon and silicon carbide may be mentioned.

The NO x purification catalysts of the present invention can be obtained by making the carrier support nanoparticles in which gold atoms and iron atoms are included in a state of close proximity.

The nanoparticles in which gold atoms and iron atoms are included in a state of close proximity can be obtained by, for example, reducing a mixture of gold salts and iron salts in the presence of a protective polymer material using a reducing agent, for example, a polyol or alcohol. The reduction reaction is performed in solution, preferably an aqueous solution, while under agitation.

At the end of the reduction reaction, the protective polymer material is separated and removed by, for example, centrifugal separation, solvent extraction, etc., and the obtained colloid in which the gold atoms and iron atoms are present in a state of close proximity is uniformly mixed with the carrier so as to ensure that the carrier supports the nanoparticles which include gold atoms and iron atoms in a state of close proximity.

The size of the AuFe particles with the gold atoms and iron atoms contained in a state of close proximity can be 0.2 to 100 nm, preferably 2 to 10 nm.

As the gold salts, aurochloric acid (HAuCl 4 ), sodium tetrachloroaurate, potassium tetrachloroaurate, gold trisodium disulfite, potassium trisodium disulfite, etc. may be mentioned.

As the iron salts, for example, iron sulfate, iron nitrate, iron chloride, iron bromide, iron acetate, iron hydroxide, etc. may be mentioned.

As the reducing polyol, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, 1,2-propanediol, dipropylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,5-pentanediol, polyethylene glycol, etc. may be mentioned. To complete the reduction of the gold ions and nickel ions, at the final stage of reduction, for example, boron dimethylamide, boron diethylamide, sodium borohydrate, borane, or another boron compound can be added as a reducing agent. As the protective polymer material, poly-N-vinylpyrrolidone(PVP), polyacrylamide, polyvinylpiridine, poly-n-vinylpyridine(PVPn), N-vinylpyrrolidone and acrylic acid copolymer, polyvinylmethylketone, poly(4-vinylphenol), oxazoline polymer, polyalkylene imine, and other polymers containing functional groups may be mentioned.

The NO x purification catalyst of the present invention is preferably composed of particles mainly comprised of Au and Fe wherein the composition of Au and Fe in the particles is Au:Fe=7:93 to 91:9 (at %). If the composition of Au and Fe in the particles is outside this range, alloy formation and the residence of gold atoms and iron atoms in a state of close proximity is difficult, interaction of gold atoms and iron atoms is not obtained, and the NO x purification performance tends to drop. In particular, even with an Au:Fe=8:92 to 26:74 (at %) ratio of composition, where alloying was believed to be difficult in conventional thin films, the formation of nanoparticles enables to take place.

The NO x purification catalyst that constitutes the present invention combines Au and Fe in close proximity and demonstrates a superior NO x purification performance. This performance cannot be matched using Au or Fe in gredients singly. In particular, the present invention demonstrates superior catalytic activity in NO x purification compared even to other alloys and Rh or other single precious metal particles.

In contrast to previous catalysts, for the NO x purification catalyst that constitutes the present invention, the temperature required to raise the NO x purification activity to a significant level does not need to be high. Even in an oxidizing atmosphere, the present invention has an NO x purification ability, so the use of fuel for making the atmosphere a reducing atmosphere becomes unnecessary or can be greatly reduced. Further, according to the NO x purification catalyst that constitutes the present invention, there is no need to lower the air-fuel ratio (A/F) in the engine. For example, during steady-state operation at a high air-fuel ratio (for example, stoichiometric, where A/F=14.7), a gasoline engine can exhibit an A/F≧20, while for a diesel engine an A/F≧30 is possible.

Below, examples of the present invention will be shown. The obtained catalysts were evaluated by the measurement methods shown below. The measurement methods shown below are illustrative. The invention is not limited to these.

1. Measurement of Alloy Composition of Catalyst

Measurement method: Measurement of composition of bulk as a whole by XRD (X-ray diffraction)

Measurement apparatus: PHILIPS X'Pert MRD

2. Measurement of Particle Shape and Particle Distribution of Alloy Nanoparticles

Measurement method 1: Measurement by TEM (transmission electron microscope)

TEM measurement apparatus: HITACHI HD-2000 STEM

Measurement method 2: HRTEM (high resolution transmission electron microscope)

HRTEM measurement apparatus: HITACHI HD-2000

3. Measurement by Elementary Analysis of Alloy Nanoparticles

Measurement method: Measurement of composition by TEM-EDS (EDS: energy dispersive X-ray spectroscopy)

TEM-EDS measurement apparatus: HITACHI HD-2000 STEM

4. Measurement of Catalytic Activity

Catalyst pellets were packed into a glass reaction tube and anchored by glass wool. A gas composed of 1000 ppm of NO and 1000 ppm of H 2 or CO and N 2 mixed in advance by balanced flow rates was run through the glass reaction tube. The gas temperature was raised at a temperature elevation rate of 20° C./min from 100° C. to 500° C. or 400° C. The NO concentration was measured by an exhaust gas analyzer (HORIBA MEXA7100H) or MS (mass spectrometry). Note that, when running a hot gas that does not include H 2 , the measurement was conducted at 500° C. after hydrogen reduction.

5. X-Ray Diffraction of AuFe Particulates

X-ray diffraction measurement apparatus: Rigaku RINT2000