Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD FOR COATING A CATALYSED PARTICULATE FILTER AND A PARTICULATE FILTER
The present invention relates to a multifunctional
catalysed engine exhaust particulate filter. In particular,
the invention is a method for the preparation of a
multifunctional catalysed particulate filter being
catalysed with a three way catalyst (TWC) and a catalyst
being active in removing nitrogen oxides by the known NH3 -
selective catalytic reduction (SCR) process, and optionally
with a catalyst having activity in the oxidation of excess
ammonia to nitrogen.
The multifunctional catalysed filter is in particular
useful for the cleaning of exhaust gas from lean burn
gasoline engines, such as the gasoline direct injection
(GDI) engine.
GDI engines generate more carbonaceous soot than gasoline
premixed injection engines. In Europe the Euro 5+ Diesel
legislation is expected to be used for GDI in the future
with a particulate mass limit at 4.5mg/km, which requires
filtration of the engine exhaust in order to reach the
above limit.
Typically, filters for use in automotive applications are
the wall flow type filter consisting of honeycombed
structured body, wherein particulate matter is captured on
or in partition walls of the honeycomb structure. These
filters have a plurality longitudinal flow channels
separated by gas permeable partition walls. Gas inlet
channels are open at their gas inlet side and blocked at
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the opposite outlet end and the gas outlet channels are
open at the outlet end and blocked the inlet end, so that a
gas stream entering the wall flow filter is forced through
the partition walls before into the outlet channels.
In addition to soot particles, exhaust gas from gasoline
engines contains nitrogen oxides (NOx), carbon monoxide and
unburnt hydrocarbons, which are chemical compounds
representing a health and environmental risk and must be
reduced or removed from the exhaust gas.
Catalysts being active in the removal or reduction of NOx,
carbon monoxide and hydrocarbons to harmless compounds are
per se known in the art.
The patent literature discloses numerous cleaning systems
comprising separate catalyst units for the removal of
harmful compounds from engine exhaust gas.
Also known in the art are exhaust gas particulate filters
coated with catalysts catalysing oxidation of hydrocarbons
and particulate matter together with selective catalytic
reduction (SCR) of NOx by reaction with ammonia being added
as such or as precursor thereof into the exhaust gas.
Multifunctional diesel particulate filters coated with
different catalysts catalysing the above mentioned
reactions are also known in the art.
In the known multifunctional filters, the different
catalysts are segmentarily or zone coated in different
zones of the filter.
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Segmentary or zone coating of different catalysts on the
filter is an expensive and difficult preparation process.
Compared to known technique, the present invention suggests
an easier method for the preparation of particulate filers
catalysed with different catalysts for the selective
reduction of nitrogen oxides with ammonia and removal of
hydrocarbons, carbon monoxide and excess ammonia.
Thus, the invention provides a method of preparation a
catalysed wall flow filter, comprising the steps of
a) providing a wall flow filter body with a plurality
longitudinal inlet flow channels and outlet flow channels
separated by gas permeable porous partition walls;
b) providing a catalyst washcoat comprising a first
catalyst composition being active in reaction of nitrogen
oxides with carbon monoxide and hydrogen to ammonia
together with a second catalyst composition being active in
selective reduction of nitrogen oxides by reaction with
ammonia to nitrogen, the first catalyst composition has a
particle size being smaller than average pore diameter of
the porous partition walls and the second catalyst
composition has a particle size with is larger than the
average pore diameter of the porous partition walls;
c) coating the filter body with the catalyst washcoat by
introduction of the washcoat into outlet end of the outlet
channels; and
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d) drying and heat treating the coated filter body to
obtain the catalysed particulate filter.
The advantage of either the first catalyst has a smaller
particle size than the mean pore diameter of the partition
walls and the second catalyst particles have a larger
particle size than the mean pore diameter of the walls is
to allow the first catalyst particles to diffuse
effectively into the partition walls and to prevent the
second catalyst from diffusing into the channels where the
specific catalytic activity is nor desired.
It is then possible to coat the filter body with different
catalysts inlet and outlet flow channels with a single
washcoat.
Useful catalyst for the reaction of Nox to ammonia by the
following reaction:
NOx +H2/C0 = NH3 +CO2 +H20
are palladium, platinum, a mixture of palladium and rhodium
and a mixture of palladium, platinum and rhodium.
These catalysts catalyse the ammonia formation under rich
burn operating conditions of the gasoline engine, i.e.
X<1. Palladium is the preferred catalyst with the highest
ammonia formation.
Ammonia being thus formed within the inlet channels by the
above reaction permeates through the partition walls of the
filter into the outlet channels and is during the rich
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operating conditions adsorbed in the SCR catalyst in the
outlet flow channels.
Both the ammonia forming catalyst and the SCR catalyst are
5 preferably deposited on the partition walls on the sides
facing the inlet channel and the outlet channel,
respectively.
In a subsequent lean burn operation cycle of the engine,
NOx being present in the exhaust gas reacts with the
ammonia stored in the SCR catalyst by the following
reaction:
NOx + NH3 = N2 + H20
As already mentioned above, SCR catalyst are per se known
in the art. For use in the invention, the preferred
catalyst being active in the selective reduction of
nitrogen oxides comprises at least one of a zeolite, a
silica aluminum phosphate, an ion exchanged zeolite, silica
aluminum phosphate promoted with iron and/or copper, one or
more base metal oxides.
A further preferred SCR catalyst for use in the invention
is a silica aluminium phosphate with chabazite structure,
such as SAPO 34, promoted with copper and/or iron.
In order to remove the excess ammonia having not reacted
with NOx, the wall flow filter comprises in an embodiment
of the invention additionally an ammonia oxidation catalyst
arranged in each outlet flow channel at least in the region
of the outlet end of the filter.
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A preferred ammonia oxidation catalyst comprises palladium,
platinum or a mixture thereof.
By contact with the ammonia oxidation catalyst, ammonia is
oxidised to nitrogen and water.
The ammonia oxidation catalyst may be deposited directly on
the partition wall in the outlet channels of the filter in
the outlet region or provided as surface layer on surface
of the SCR catalyst layer.
The invention provides additionally a method of preparation
of a catalysed wall flow filter.
In its broad embodiment the invention provides a of
preparation a catalysed wall flow filter, comprising the
steps of
a) providing a wall flow filter body with a plurality
longitudinal inlet flow channels and outlet flow channels
separated by gas permeable porous partition walls;
b) providing a catalyst washcoat comprising a first
catalyst composition being active in reaction of nitrogen
oxides with carbon monoxide and hydrogen to ammonia and a
second catalyst composition being active in selective
reduction of nitrogen oxides by reaction with ammonia to
nitrogen, the first catalyst composition has a mode
particle size being smaller than average pore diameter of
the porous partition walls and the second catalyst
composition has a mode particle size being larger than the
average pore diameter of the porous partition walls;
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c) coating the filter body with the catalyst washcoat by
introduction of the washcoat into outlet end of the outlet
channels; and
d) drying and heat treating the coated filter body to
obtain the catalysed particulate filter.
Specific catalyst compositions for use in the invention are
mentioned hereinbefore and further disclosed in claims 2 to
4.
In further an embodiment of the invention, the filter is
additionally coated with a so called ammonia slip catalyst,
which is a catalyst being active in the oxidation of excess
of ammonia to nitrogen and water.
Thus in this embodiment the inventive method comprises the
steps of
providing a second washcoat containing a catalyst
composition being active in the selective oxidation of
ammonia; and
coating at least a part of the outlet channels with the
washcoat subsequently to the coating with the catalyst
washcoat.
When preparing the washcoats for use in the invention, the
catalysts being usually in particle form are milled or
agglomerated to the required particle size and suspended in
water or organic solvents, optionally with addition of
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binders, viscosity improvers, foaming agents or other
processing aids.
The filter is then washcoated according to common practice,
including applying vacuum in the filter, pressurizing the
washcoat or by dip coating.
The amount of the first catalyst coated on the filter is
typically 10 to 140 g/l, and the amount of the second
catalyst on the filter is typically 10 to 100g /1. The
total catalyst loading on the filter is typically in the
range of 40 to 200 g/l.
Examples of suitable filter materials for use in the
invention are silicon carbide, aluminium titanate,
cordierite, alumina, mullite or combinations thereof.
Example
A suspension of the first catalyst composition is in a
first step prepared from a powder mixture of palladium
rhodium deposited on cerium oxide and alumina particles of
a particle size smaller than the filter wall mean pore
size.
A suspension of the mixture first catalyst is prepared by
mixing 20 g of these powders in 40 ml demineralised water
pr liter filter. A dispersing agent Zephrym PD-7000 and an
antifoam agent are added. The suspension is milled in a
bead mill. The particle sizes of the final suspension must
be smaller than the mean pore diameter of the pores in the
wall of the wall flow filter
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A suspension of a second catalyst is made by mixing and
dispersing 100 g of silica aluminium phosphate SAPO-34
promoted with 2% copper in 200 ml demineralised water pr
liter filter. A dispersing agent Zephrym PD-7000 and an
antifoam agent are added. The particle sizes must be larger
than the mean pore diameter of the pores in the wall of the
wall flow filter
The suspensions of the first catalyst and the second
catalyst are then mixed to one suspension.
A high porosity (approximately 60% and wall mean pore size
approx 18 pm) conventionally plugged SiC wall flow filter
is used.
The mixed suspensions of first and the second catalyst is
washcoated from the filters outlet end of the filters
permeate side by standard washcoat methods permeate side,
dried and calcined at 750 C
