Catalyst – solid sorbent – or support therefor: product or process – Miscellaneous
Reexamination Certificate
2001-07-23
2004-06-22
Silverman, Stanley S. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Miscellaneous
C502S400000
Reexamination Certificate
active
06753294
ABSTRACT:
This invention relates to a wall-flow filter, and in particular to a wall-flow filter including a catalyst.
Exhaust gases of a combustion engine contain a number of components linked with damaging health and the environment. One of these is the soot component. A way of controlling the amount of soot in the exhaust gas is to use a filter downstream from the exhaust manifold, the idea being to burn (oxidise) soot trapped on the filter, thereby regenerating the filter.
A known type of soot filter is the wall-flow filter. This filter can be made, for example, from a ceramic monolith including channels in a honeycomb arrangement. A typical embodiment has each channel plugged at one or other end thereof, and at the opposite end to the laterally and vertically adjacent channels. When viewed from either end, the alternately plugged and open ends of the channels take on the appearance of a chessboard. The ceramic material from which the filter can be made has a pore size sufficient to allow gas permeability so that the pressure drop across the filter is relatively low, but which prevents the passage of soot. Thus soot is filtered from the exhaust gases.
EP-A-0341832 and corresponding case U.S. Pat. No. 4,902,487 describes a process and treatment system for soot-containing exhaust gas, the gas also containing nitric oxide (NO), which process comprising passing the gas unfiltered over an oxidation catalyst to convert NO to nitrogen dioxide (NO
2
), collecting the soot on a downstream filter and combusting the collected soot continuously at under 400° C. by reaction with the NO
2
; and there have been recent proposals to add further steps to that process and system, for example nitrogen oxides (NOx) removal steps (see EP-A-0758713). EP-A-0341832 and U.S. Pat. No. 4,902,487 describe Johnson Matthey's Continuously Regenerating Trap (CRT™) technology and are incorporated herein by reference.
In the process described in EP-A-0341832 the oxidation step and the filter combustion step are carried out in two different honeycombs each in a separate shell or can or mounted within a single can. However, there are problems in adopting either embodiment. A problem with the former embodiment is that there can be limited space under-floor on a vehicle to mount each can. In the latter embodiment, a problem is that the construction of the can is complicated. If further downstream process steps are required these problems are exacerbated.
We have now found that these and other problems can be overcome or reduced by carrying out each of the treatment steps on a single wall-flow filter or single “brick”. U.S. Pat. No. 5,089,237 discloses a soot burn-off filter for an exhaust system of a combustion engine, which filter includes a porous ceramic honeycomb block having channels plugged alternately at the ends to define a flow path through the partition walls of the channels, the walls at the inlet end having a catalytic coating. We understand from this document that the filter is used in a discontinuous process in which soot is allowed to accumulate on the filter and is periodically burnt off by raising the temperature and ensuring that sufficient oxygen (O
2
) is available. Owing to the combustion of a substantial quantity of soot in a relatively small space, -temperatures high enough to produce destructive effects on the filter are readily attained. To limit such effect the filter of U.S. Pat. No. 5,089,237 provides a gas-tight region in the partition walls at the downstream end of the upstream channels. Although the catalytic coating is stated to lower the temperature at which soot combustion takes place, it evidently does not make the gas-tight region unnecessary.
According to one aspect, the invention provides a wall-flow filter for an exhaust system of a combustion engine, which filter comprises: a plurality of channels in honeycomb arrangement, wherein at least some of the channels are plugged at an upstream end and at least some of the channels not plugged at the upstream end are plugged at a downstream end; an oxidation catalyst on a substantially gas impermeable zone at an upstream end of the channels plugged at the downstream end; and a gas permeable filter zone downstream of the oxidation catalyst for trapping soot, characterised in that in an exhaust system the oxidation catalyst is capable of generating sufficient NO
2
from NO to combust the trapped soot continuously at a temperature less than 400° C.
The term “continuously” means that collected soot is combusted in a continuous exhaust gas flow; it thus excludes (except in the event of malfunction) the combustion of a large amount of accumulated soot. It does, however, encompass relatively minor variations in the level of collected soot and in the gas composition in response to normal variations of engine operating conditions and to short-term injection of reductant or NOx specific reactants in order to remove NOx downstream of the filter.
For the avoidance of doubt, by “absorb” herein, we mean to hold a relevant species on a relevant surface of a body (otherwise “adsorb”) and/or to hold a species below the surface of a body i.e. within the body.
Exhaust gases from a combustion engine also contain unburnt hydrocarbon (HC), carbon dioxide (CO
2
), carbon monoxide (CO), steam (H
2
O(g)) and nitrogen (N
2
). Its content of NOx comprises NO and NO
2
, the majority being NO. At least sufficient NOx should be present to provide, after the catalytic oxidation of NO to NO
2
, at least enough NO
2
to oxidise the collected soot as it is formed, or after a small accumulation of soot. As described in the above mentioned EP-A-0341832, and in PCT application no. GB00/02062, additional NOx may be supplied by e.g. injecting nitric acid or the product of local oxidation of ammonia (NH
3
) or an ammonia precursor, such as urea.
The filter of the present invention provides a gaseous flow path defined in part by the open ends of each channel plugged at the downstream end, and the pores of the gas permeable wall of the channel. Soot in the gaseous exhaust enters the channel, but cannot pass through the pores of the channel wall. Thus soot is filtered from the gaseous exhaust by the pores of the filter. Furthermore, the gaseous exhaust is forced to flow over the entire zone supporting the oxidation catalyst, as the wall of the filter on which it is supported is substantially gas impermeable. This achieves the best possible yield of NO
2
from NO at a given temperature and also avoids caking the catalyst in soot.
Preferably, the arrangement of channels plugged at the upstream and downstream ends of the filter is such that each channel of the filter is plugged at one or other end thereof, and at the opposite end to the laterally and vertically adjacent channels, although other configurations can be used. For example, in an arrangement of striated appearance, a first longitudinal array of channels is plugged at one end, and the longitudinal arrays of channels either side of the first array are plugged at the opposite ends of the filter, and so on. The provision of a filter including at least some unplugged channels, thereby to act as a by-pass to the filter channels, is also within the ambit of the present invention. The unplugged channels can also be used to introduce a reactant required downstream of the wall-flow filter zone.
The channels are preferably square in cross-section but can be any other shape such as circular, rectangular, hexagonal or triangular.
Preferably, the oxidation catalyst, which is advantageously a compound including a platinum group metal (PGM), such as platinum (Pt) or palladium (Pd), blocks the filter pores in the gas impermeable zone, thereby making the zone substantially gas impermeable. Filter-grade materials for making the filter suitably have a mean pore diameter in the range 0.4 to 20×10
−3
inch (1 to 50 &mgr;m) for gas treatments at about atmospheric pressure, but other values may be more appropriate for liquids or at higher or lower pressures. Although it is possible to manufacture a filter in which the filter mater
Brisley Robert James
Twigg Martyn Vincent
Wilkins Anthony John Joseph
Johnson Edward M.
Johnson Matthey Public Limited Company
RatnerPrestia
Silverman Stanley S.
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