EXHAUST CATALYSTSThe present invention relates to exhaust catalysts and in particular to exhaust catalysts based on a support structure.
Catalysts for the exhaust systems of motor vehicles may be based on a support structure. The support structure defines a plurality of channels extending axially through the structure from one end to the other, the channels forming paths for the flow of exhaust gasses.
The support structure may be of monolithic form made from ceramic materials such as cordierite, cordierite-aalumina, silicone nitride, zircon mullite, spodumene, alumina-silica-magnesia, zirconium silicate, sillimanite, magnesium silicates, zircon, petalite, a-alumina and alumino silicates. Preferably these materials are in crystaline form so that the walls defining the channels of the support structure are porous.
Alternatively, the support structure may be made of metals of monolithic form similar to the ceramic structures described above or formed from corrugated metal sheets.
Conventionally, the support structure is coated with a carrier layer of catalytically active metal oxide, for example y-alumina with relatively high porosity. A metal catalyst, for example platinum and/or rhodium is then deposited in the pores of the carrier layer.
Conventionally the distribution of these metal catalysts are substantially uniform throughout the catalyst structure.
Catalytic structures of the form described above are located in the exhaust system of the motor vehicle. In order to effectively remove all noxious pollutants from the exhaust gases, the catalytic structure will typically be of the order of 100 mm long. In order to achieve an acceptable back pressure, the cross-sectional area of the catalytic structure must be considerably increased in relation to the cross-sectional area of the exhaust pipe.
For example, for an exhaust system having an exhaust pipe of 50 mm diameter, the catalyst structure will typically be of elliptical cross-section having a major diameter of 185 mm and a minor diameter of 90 mm. Because of this significant difference in cross-sectional areas of the exhaust pipe and catalytic structure, flow of exhaust gases through the catalytic structure are concentrated through the portion thereof in the direct flow path from the exhaust pipe while areas of the catalytic structure more remote from the direct flow path have low flow rates. In order to accommodate the high flow rates through the central portion of the catalytic structure, the gas passing through the channels in this portion of the catalytic structure will be moving faster than the gas flowing through the channels in the outer portions of the catalytic structure and consequently will have less time to react with the catalyst. Furthermore, due to the high flow rates through the central portions of the catalytic structure, the catalyst in this region will be deactivated at a faster rate than the catalyst in the outer regions.
According to the present invention an exhaust catalyst comprises a support structure, said support structure being mounted within a catalyst-container, the container having an inlet and an outlet, the support structure being mounted within the container between the inlet and outlet so that exhaust gases entering the container by the inlet must flow through the support structure to reach the outlet, the cross-sectional area of the inlet being substantially smaller than the cross-sectional area of the support structure, said support structure defining a plurality of channels extending axially from one end of the support structure to the other, a carrier coating being applied to the walls of the channels and a metal catalyst being deposited on the carrier coat, the metal catalyst being deposited at higher concentrations in the axially extending portion of the support structure in the direct flow path from the inlet and at lower concentrations in axially extending portions of the support structure more remote from the direct flow path from the inlet.
In this manner, the amount of catalytic metal available to treat the faster flowing exhaust gases is increased thereby overcoming the problems of reduced reaction time and increased deactivation in those areas, without increasing the overall amount of catalytic metal used and hence the cost of the exhaust catalyst.
According to a preferred embodiment of the present invention the ratio of the concentration of metal catalyst in portions of higher concentration to that in portions of lower concentration is from 6:5 to 6:1. The concentration of metal catalyst in the regions of higher concentration may be from 0.00175 gm/cc (50gm/ft3) to 0.0035 gm/cc (lOOgm/ft3) and the concentration of metal catalyst in the regions of lower concentration may be from 0.0007 gm/cc (20gm/ft3) to 0.00175 gm/cc (50 gm/ft3).
According to a further aspect of the present invention a method of forming an exhaust catalyst as hereinbefore defined includes depositing a metal catalyst on a support structure, the support structure defining a plurality of channels extending axially from one end of the support structure to the other, the metal catalyst being deposited so that in selected axially extending portions of the support structure the concentration of metal catalyst is higher than in other axially extending regions of the support structure.
The carrier coating may be applied to the support structure in any suitable manner. Hitherto, this has been achieved by dipping the support structure into a slurry of the coating material. Alternatively, a slurry of the coating material may be poured over one end of the support structure, so that it washes through the channels, depositing the coating material on the walls of the channels. The support structure is then dried and the coating layer calcined by heating to a temperature between 1500 and 8000C.
The metal catalyst may also be applied to the support structure, by pouring a solution of salts of the metal catalyst over the end of the support structure and allowing it to wash through the channels and subsequently reducing the salts by heating. In order to obtain the differential dispersion of the metal catalyst throughout the support structure, this operation may be carried out in several passes, the regions of low metal catalyst concentration being masked off for some passes, so that lower concentrations of the metal catalyst will be deposited. Alternatively, the solution of salts of the metal catalyst may be sprayed onto the end face of the catalyst the concentration of the solution or the rate or duration at which the solution is sprayed onto different areas of the end face may be varied to vary the deposition of the metal catalyst in different regions of the catalyst.
An embodiment of the invention is now described, by way of example only, with reference to the accompanying drawings which illustrate in cross-section, an exhaust catalyst in accordance with the present invention.
As illustrated, an exhaust catalyst 10 comprises a monolithic catalyst support structure 11 formed from cordierite. The support structure 11 is 101.6 mm long and of elliptical cross-section having a major diameter of 185 mm and a minor diameter of 90 mm. The support structure 11 is located in a container 12 of similar internal cross-sectional dimensions. The container 12 has an inlet 13 on one side of the catalyst 10 and an outlet on the side of the catalyst 10 axially remote from the inlet 13. The support structure 11 is located adjacent the inlet 10. The container 12 may be longer than the support structure 11 and may contain silencing baffles through which the exhaust gases must travel after passing through the catalyst 10 and before exiting the container 12 through the outlet. The inlet 13 is 57.15 mm in diameter and enters the container 12 coaxially of the container 12 and the support structure 11 located therein.
Because of the difference in cross-sectional areas of the inlet 13 and support structure 11, the flow of exhaust gases is concentrated through the cross-sectional area A of the support structure 11.
The support structure 11 defines a matrix of channels or cells which extend longitudinally from one end face of the support structure 11 to the other. The support structure 11 defines of the order of 60 channels per square centimetre of the end face, each channel being separated from adjacent channels by walls of a thickness of the order of 0.165 mm.
The walls of the channels are coated with a porous layer of calcined y-alumina. This coating is produced by pouring a slurry of alumina over the end face of the catalyst structure and allowing it to wash down through the channels. This is done in a controlled manner, so as to ensure a uniform coating, by passing the support structure standing on end, through a waterfall of the yalumina slurry.
After depositing the y-alumina on the support structure, the support structure is dried and the layer of y-alumina calcined by heating.
A catalytic layer containing five parts platinum to one part rhodium is then deposited on the porous layer of yalumina. This is done by pouring a solution of platinum and rhodium salts over the end face of the support structure 10 in similar manner to that in which the coating of y-alumina was formed. The metal catalyst is however deposited in several passes. For the initial passes, the metal catalyst solution is passed over the whole area of the end face of the support structure 11.
When the concentration of metal catalyst in the support structure 11 reaches 0.0007 gm/cc (20 gm/ft3) the end face of the support structure outside the area A, is masked. Further passes are then made depositing further metal catalyst on only the channels open to the area A of the end face of the support structure 11, until concentration of the metal catalyst in this area A is 0.0021 gm/cc (60 gm/ft3). The catalyst structure is subsequently baked in an oven to reduce the salts to the metals.
An exhaust catalyst 10 is thereby built up in which the concentration of metal catalyst in the regions of the catalyst subjected to high exhaust gas flow are substantially higher than those in regions of low gas flow. In this manner, the exhaust catalyst 10 can accommodate the different flow rates in the different regions and also the life of the catalyst may be prolonged without increasing the amount of metal catalyst used and hence the cost of the catalyst.
Various modifications may be made without departing from the invention. For example, it will be appreciated that the regions of high and low catalyst concentrations may be varied to accommodate the gas flows through a particular exhaust system. Furthermore, rather than having just two regions of varying metal catalyst concentration, the metal catalyst concentration may be varied in several graduations or even continuously, between the high concentration and low concentration regions.