Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Silicon containing or process of making
Reexamination Certificate
1998-11-30
2002-07-16
Wood, Elizabeth D. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Silicon containing or process of making
C502S232000, C502S240000, C502S250000, C502S258000, C502S262000, C502S263000, C502S527120, C422S177000
Reexamination Certificate
active
06420306
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a gas purifying catalyst, and, more particularly, to a burned gas purifying catalyst for use with an exhaust system of an automobile engine suitable for emission level controls of nitrogen oxides (NOx), hydrocarbons (HC) and carbon monoxide (CO).
2. Description of the Related Art
As one of catalysts installed in an exhaust line of an automobile engine to purify the exhaust gas or to significantly lower emission levels of oxides of nitrogen (NOx) as well as hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gas an automobile exhaust control catalyst, there has been a monolith type catalytic convertor which is formed with an under catalyst layer having active alumina particles and platinum (Pt) and rhodium (Rh) on a monolith honeycomb carrier and an over catalyst layer, coated over the under catalyst layer, which comprises barium-fixed ceria (cerium oxide) particles, active alumina particles and palladium (Pd). Such a catalyst is known from, for instance, Japanese Unexamined Patent Publication No. 3-207446. The reason for fixing barium (Ba) to the ceria particles is to prevent the ceria from suffering heat deterioration. The barium-fixed ceria particles are produced in such a manner to dry and solidify a mixture of a barium nitrate solution with ceria particles as a solid lump of barium nitrate-adsorbed ceria particles and break it into particles. The over catalyst layer is coated by dipping a catalyst carrier in a slurry of a palladium chloride solution with the barium nitrate-adsorbed ceria particles and, thereafter, dry and burn the slurry film on the catalyst carrier.
While barium (Ba) is essentially intended to play as an agent to prevent heat deterioration of ceria, it is in some cases used to purify exhaust gases, in particular to reduce nitrogen oxides (NOx) in exhaust gases as is known from, for instance, Japanese Unexamined Patent Publication No. 7-108172. The catalyst described in the Japanese Unexamined Patent Publication No. 7-108172 is a monolith honeycomb type catalytic convertor that carries an under catalyst layer having barium (Ba) supported by an alumina support and an over catalyst layer having platinum (Pt) and rhodium (Rh) supported by an alumina support. This catalyst reduces nitrogen oxides (NOx) through the steps of oxidizing nitrogen oxides (NOx) with the barium (Ba) in the over catalyst layer, a lowering the concentration of oxygen (O
2
) in the exhaust gas so as to produce a reducing atmosphere in which the nitrogen oxides (NOx) are separated from the barium, and reducing the nitrogen oxides (NOx) by the catalytic metal in the under catalyst layer making the utilization of hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gas as reducing agents.
A typical problem the exhaust gas emission control catalysts experience is sulfur (S) poisoning and/or water (H
2
O) poisoning and is significant in particular if they contain large amounts of barium which has a strong tendency to be poisoned. It has been proved by the inventors of this invention that lanthanum (La) has a tendency of the sulfur (S) poisoning and/or water (H
2
O) poisoning as well. Accordingly, unless the catalyst containing platinum and rhodium or palladium as main catalytic metals is kept away from the sulfur (S) poisoning and water (H
2
O) poisoning, it is difficult that the catalyst maintains its intended emission control efficiency for a long period of time.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a catalyst construction for purifying gases which prevents lanthanum and barium from sulfur poisoning and/or water poisoning.
It is another object of the present invention to provide an catalyst construction for purifying automobile exhaust gases which maintains its intended emission control efficiency for a long period of time.
This invention has been achieved on the basis of the knowledge obtained from the results of various investigations and assessment conducted by the inventors of this application that a combination of a composition of barium and lanthanum and a zeolite support prevents the barium and lanthanum from sulfur poisoning and/or water poisoning and maintains the intended activity of the barium and lanthanum for a long period of time.
These objects of the present invention are achieved by providing a gas purifying catalyst construction comprising an under catalyst layer containing at least one of barium and lanthanum and an over catalyst layer containing an agent to absorbing water in a gas. The over catalyst layer prevents the barium and/or the lanthanum from sulfur poisoning and/or water poisoning. The catalytic metal may be contained either one or both of the under and over catalytic layers.
The water adsorbing agent comprises a crystalline metal silicate which works to prevent the barium and lanthanum from sulfur poisoning and/or water poisoning. This is because the metal silicate, such as MFI-type zeolite, is one of materials that exhibit excellent water adsorbing performance and prevent water poisoning, consequently. Further, the metal silicate in the over catalyst layer prevents the barium and lanthanum in the under catalyst layer from easily contacting with gases, enhancing the prevention of sulfur poisoning and/or water poisoning.
The over catalyst layer contains one or more selected from a noble metal group of catalytic materials such as platinum (Pt), rhodium (Rh), palladium (Pd) and iridium (Ir), which works to lower emission levels of oxides of nitrogen (NOx) as well as hydrocarbons (HC) and carbon monoxide (CO) in burned gases. In particular, when the over catalyst layer contains platinum and rhodium as the noble metal group of catalytic materials, the catalyst exhibits an excellent NOx emission control efficiency in burned gas resulting from the combustion of a lean air-fuel mixture through a synergistic effect of the platinum and rhodium catalytic materials and additives such as barium or lanthanum in combination.
If a small amount of palladium is added into the under catalyst layer, it is preferred to support the palladium by a cerium oxide or alumina so as to deposit the palladium particles separately from the rhodium particles. The reason for a decline in the catalytic activity with a rise in ambient temperature has been considered to be caused by the absence of intermediate products of the hydrocarbon combustion contributory to reduction or decomposition of nitrogen oxides which results from expeditious combustion of hydrocarbons. Although the reason for the improvement in high temperature NOx emission control efficiency of the catalyst resulting from the presence of the cerium oxide has not been clearly proved, the presence of cerium in the catalyst of the invention suppresses the combustion of hydrocarbons at high temperatures and produces easily intermediate products of the hydrocarbon combustion. When the cerium is contained in the under catalyst layer, the cerium is prevented by the barium and lanthanum from experiencing thermal deterioration and exhibits its primary chemical activity for a long period of time.
The under catalyst layer may contain an additive of palladium or alumina with the effect of improving low temperature catalytic activity of the catalyst. The palladium exhibits its catalytic activity at temperatures lower as compared with platinum and rhodium and burns hydrocarbons in low temperature exhaust gases from an automobile engine which is still cold. Consequently, even when the exhaust gas is still at low temperatures, the combustion of hydrocarbons by means of the under catalyst layer causes the over catalyst layer to rapidly raise its temperature sufficiently to burn hydrocarbons. The combustion of hydrocarbons is utilized to reduce or decompose nitrogen oxides in burned gases.
Contact of the palladium in the under catalyst layer with a large amount of hydrocarbons causes poisoning, lowering the catalytic performance. However, the metal silicate contained in the over catalyst layer absorbs hydrocarbons in bur
Iwakuni Hideharu
Kurokawa Takahiro
Kyogoku Makoto
Murakami Hiroshi
Okamoto Kenji
Mazda Motor Corporation
Wood Elizabeth D.
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