Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
1997-03-31
2001-01-30
Niebling, John F. (Department: 2812)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S340000, C502S341000
Reexamination Certificate
active
06180558
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a catalytic material for an exhaust gas purifying catalyst, an exhaust gas purifying catalyst for use with an internal combustion engine in which the catalytic material is used, and a method of the exhaust gas purifying catalyst.
2. Description of Related Art
In order for automotive vehicles to purify exhaust gases, it has been known to install in an exhaust line a honeycomb type of exhaust gas purifying catalyst which comprises a honeycomb carrier and a catalyst layer coated on the honeycomb carrier. The exhaust gas purifying catalyst coating layer contains therein ceria (i.e. cerium oxide CeO
2
) particles with barium (Ba) fixed to catalytic particles, activated alumina particles and noble metal components. Such a catalyst is known from, for example, Japanese Unexamined Patent Publication No. 3-207446.
The barium (Ba) is fixed to the ceria particles with an effect of preventing thermal degradation of the ceria particles. In order to firmly fix barium to the ceria particles, ceria powders are mixed with a barium nitrate solution and the mixture is dried. A solid lump of barium nitrate adsorbed-ceria is crushed to particles or powders. The barium nitrate adsorbed-ceria powders are mixed with a solution of palladium chloride and water to prepare slurry of exhaust gas purifying catalyst. After having immersed a catalyst carrier in the slurry and formed a catalyst layer containing barium-fixed ceria and palladium on the catalyst carrier. Finally, the catalyst layer is baked and dried.
While the barium contained in the catalyst has a function of preventing thermal degradation of the ceria, it has been known in the art that the barium is actively utilized for purifying exhaust gases and, in particular, for decomposing nitrogen oxides (NOx) in exhaust gases. That is, in the event where the concentration of oxygen in an exhaust gas is high resulting from lean burning, barium (Ba) supported together with catalytic noble metals such as platinum (Pt) by a alumina supporting material or the like is made to absorb nitrogen oxides (NOx) oxidized with the catalytic noble metals so as to temporarily lower the concentration of oxygen in the exhaust gas and consequently create a reducing atmosphere which removes nitrogen oxides from the barium. In the reducing atmosphere, the catalytic noble metals decompose the removed nitrogen oxides (NOx) by utilizing hydrocarbons (HC) and carbon monoxide (CO) as reduction agents in the exhaust gas.
If barium itself contained in a catalyst encounters thermal degradation, the barium is hard to perform the function of preventing thermal degradation of ceria contained in the catalyst and the function of absorbing nitrogen oxides. When barium is exposed to a high temperature of exhaust gas, the barium dissolves and diffuses in and binds to an adjacent supporting material such as alumina particles. In some cases, the barium forms composite compounds (composite oxides) which causes a decline in the primary functions of barium and, in addition, a decline in the activity of catalytic noble metals.
Japanese Unexamined Patent Publication No. 3-2074461 describes another type of an exhaust gas purifying catalyst. That is, slurry of a mixture of a barium nitrate solution mixed with ceria (CeO
2
) and boehmite (Al
2
O
3
. H
2
O) and a noble metal solution is applied to a honeycomb carrier. This catalyst coating layer is dried at 130° C and there after baked at 550° C.
Because boehmite includes a portion of alumina Al
2
O
3
, barium (Ba) dissolves and diffuses in and firmly fixes to aluminum (Al) when the catalyst coating layer is exposed to a high temperature of heat.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a catalytic base material for a exhaust gas purifying catalyst which prevents the exhaust gas purifying catalyst from encountering thermal degradation and aggravation of NOx purifying efficiency, and a method of producing the catalytic base material.
This invention has been achieved on the basis of the acknowledge obtained from the results of various experiments and investigations conducted by the inventors of this application that, when a mixture of barium oxides and ceria particles is heated to a relatively high temperature to intentionally create and accumulate coarse-grained barium oxides on surfaces of the ceria particles, the ceria particle and barium oxides are firmly bound, and there are produced no composite compounds like when alumina is used as a catalytic base material, and that consequently the barium does not encounter aggravation of its functions, and that the barium is prevented from dissolving and diffusing in alumina particles or from forming compounds with alumina.
The object of the invention is accomplished by providing a catalytic base material for an exhaust gas purifying catalyst comprising ceria and barium oxides created on surfaces of the ceria particles by heating and fixed onto the ceria particle surfaces. Because the barium oxides are created on the ceria particle surfaces by heating the barium, the barium oxides are firmly fixed to the ceria particles and consequently are prevented from removing from the ceria particles, from dissolving and diffusing in other solid materials or from creating composites even when the catalytic base material is heated to a high temperature until the barium oxides are heated to a temperature high enough to decompose. The catalytic base material is utilized as a supporting material for catalytic metals, and also as an additive for an exhaust gas purifying catalyst. The barium and ceria are made to function as a heat resistant assisting or supplementary catalyst for the catalytic metals.
The catalytic base material for an exhaust gas purifying catalyst is produced by a method including the steps of providing a mixture of a barium solution and cerium contained powders indissoluble in the barium solution, and heating the mixture at a temperature between approximately 400 and 1100° C. to produce barium oxides accumulated on and firmly fixed to surfaces of the ceria particles. Because mixing the cerium contained powders with the barium solution causes barium to adhere evenly to cerium contained powders, barium oxides are created and accumulated on surfaces of ceria particles by heating the mixture. It is essential to heat the mixture at a temperature higher than 400° C. for creation and accumulation of barium oxides. Further, in order to prevent creation and accumulation of excessively coarse-grained barium oxides and ceria particles with decreased specific surface areas which results in degradation of the barium oxides and ceria particles, it is also essential to keep the temperature of heat treatment less than 1100° C. The catalytic base material supporting catalytic metals in an exhaust gas purifying catalyst fulfil effectively the intended function of preventing thermal degradation of the exhaust gas purifying catalyst, adsorption of nitrogen oxides (NOx) and oxygen storage as an assisting or supplementary catalyst. The catalytic base material may be mixed with alumina as a catalytic metal supporting material. In an exhaust gas purifying catalyst having the mixture, catalytic metals are distributed between and supported by the catalytic base material as a first supporting material and the alumina as a second supporting material. This exhaust gas purifying catalyst is made to contain the most part of barium supported by the ceria and is hard to contain the barium supported by the alumina. The exhaust gas purifying catalyst thus produced prevents barium from dissolving and diffusing in the alumina or from oxidizing with the alumina to create compound composites owing to the function of the barium even when it is exposed to a high temperature of heat. An exhaust gas purifying catalyst having a catalytic layer comprising the catalytic base material, alumina and catalytic metals is produced by a method including the steps of preparing slurry of the catalytic base material and alumina, coating the slurry on
Iwakuni Hideharu
Kawakami Yoko
Kurokawa Takahiro
Kyogoku Makoto
Murakami Hiroshi
Ghyka Alexander G.
Mazda Motor Corporation
Niebling John F.
Nixon & Peabody LLP
Studebaker Donald R.
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