Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to a catalytic automotive
emission control process with improved hydrocarbon
suppression during the cold start phase using a threeway
catalyst known per se and a hydrocarbon adsorber which is
arranged before the catalyst in the exhaust gas stream and
which, after cold starting, adsorbs hydrocarbons present in
the exhaust gas until the three-way catalyst has reached its
full operating temperature and efficiency and only desorbs
them to the exhaust gas after heating so that the desorbed
hydrocarbons can be converted into harmless components by the
now relatively active three-way catalyst.
The fu,~ure limits for automotive pollutant emissions are
laid down in the regulations TLEV/1994 and LEV/1~97 (LEV =
low emission vehicle). They represent a significant
tightening of the limits, particularly for hydrocarbons.
Since modern automotive emission control catalysts have
reached a high level of pollutant conversion in their
operationally warm state, it will only be possible to keep to
future limits by improving pollutant conversion during the
cold start phase. This is be~ause a large part of the total
pollutants released is emitted during the cold start phase of
the legally stipulated test cycles (e.g. US FTP 75), because
in this phase the catalysts have not yet reached the
operating temperature of 300 to 400~C required for conversion
of the hydrocarbons.
Emission control systems consisting of a hydrocarbon
adsorber and a following catalyst have already been proposed
with a view to reducing emissions during
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the cold start phase. The function of the hydrocarbon
adsorber in these systems is to adsorb the hydrocarbons
- present in the exhaust gas at the relatively low temper-
atures prevailing during the cold start phase. It is
only after fairly significant heating of the adsorber
that the hydrocarbons are desorbed and pass with the now
relatively hot exhaust gas to the catalyst already close
to its operating temperature where they are effectively
converted into harmless water and carbon dioxide. One
of the key requirements which the adsorber is expected
to satisfy is that it should be able to adsorb hydrocar-
bons preferentially to the steam already present in
abundance in the exhaust gas.
The disadvantage of this known solution is that
the desorption of the hydrocarbons actually begins at
temperatures around 250~C so that optimal conversion
still cannot take place on the following catalyst. In
addition, the adsorber is in danger of being destroyed
by heat because it has to be installed in the exhaust
system near the engine and, accordingly, is exposed to
temperatures of up to 1000~C in long-term operation.
Numerous proposals have been put forward in the patent
literature with a view to overcoming this problem, for
example in DE 40 08 789, in EP 0 460 542 and in US
5,051,244. These documents also start out from a
combination of a hydrocarbon absorber and a catalyst,
but propose elaborate circuits for the exhaust gas to
overcome the described disadvantages.
Thus, according to US 5,051,244, the actual
catalyst is preceded by a zeolite adsorber which adsorbs
the pollutants, particularly hydrocarbons, in the
exhaust gas in the cold state and releases them again
with increasing heating of the exhaust system. The
adsorber is protected against destruction by overheating
in long-term operation of the engine by switching on a
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short circuit line from the engine directly to the catalyst.
During the first 200 to 300 seconds after starting, the
entire exhaust gas is passed over the adsorber and the
catalyst. In this operational phase, the hydrocarbons are
taken up by the adsorber. The adsorber and the catalyst are
increasingly heated by the hot exhaust gas. The adsorber is
short-circuited if, through an increase in temperature,
desorption begins to overtake adsorption. The exhaust gas
then flows directly over the catalyst. On reaching the
operating temperature, part of the hot exhaust gas is passed
over the ad~rber until the pollutants have been completely
desorb~d 50 that they can then be efficiently converted by
the catalyst. After desorption, the adsorber is short-
circuited again so that it is protected against thermal
overloading.
A Y-zeolite with an Si to Al atomic ratio of at least
2.4 is pxoposed as adsorber in US 5,051,244. The zeolite
adsorber may contain fine-particle catalytically active
metals, such as platinum, palladium, rhodium, ruthenium and
mixtures thereof.
These solutions known from the prior art are technically
very complex, expensive and susceptible to damage. The
present invention provides a process by which the
disadvantages known from the prior art are eliminated or at
least mitigated and ensures very good hydrocarbon suppression
during the cold start phase.
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More particularly, there is provided a catalytic
automotive emission control process with improved hydrocarbon
suppression during the cold start phase using a three-way
catalyst known per se and a hydrocarbon adsorber which is
arranged before the catalyst in the exhaust gas stream and
which, after cold starting, adsorbs hydrocarbons present in
the exhaust gas until the three-way catalyst has reached its
full operating temperature and efficiency and only releases
them to the exhaust gas after heating so that the desorbed
hydrocarbons can be converted into harmless components by the
now relatively active three-way catalyst. The process
according to the invention is characterized in that the
hydrocarbon adsorber has a greater specific heat capacity
than the following three-way catalyst.
In the context of the invention, the specific heat
capacity is not the specific heat of substances, but rather
the heat capacity per unit volume of the adsorber or catalyst
element. Adsorbers or catalysts may be present in the form
of loose layers of granules, extrudates or pellets or in the
form of monolithic foams or honeycombs. To calculate the
specific heat capacity in the context o~ the invention, the
heat capacity of these layers or monoliths is based on the
geometric volume of the layers and elements, including all
voids and pores. Accordingly, the specific heat capacity is
not a material parameter in the physical sense, rather it is
dependent upon the macroscopic form and the microscopic
structure of the adsorber or catalyst. Accordingly,
possibilities for influencing the specific heat capacity are
available to the expert in the choice of the material for the
adsorber or catalyst and in the processing and geometric
configuration and, accordingly, leave the expert with
considerable scope for carrying out the present invention.
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The specific heats of a few materials suitable as
starting materials for the manufacture of supports for
5 adsorber and catalyst coatings are mentioned by way of
example below:
Table 1
Material Density Specific heat
[g.cm 3] tJ-g-l.K-l]
Alpha-aluminium oxide 3.97 1.088
Mullite 2.80 1.046
Zirconium oxide, stabil. 5.70 0.400
Stainless steel, highly
alloyed (20Cr; 7Ni) 7.86 0.544
As Table 1 shows, the specific heats of suitable
materials cover a range of 0.400 to 1.088 J.g l.K 1, Taking
into account the density of the particular materials, the
expert is left - solely through the choice of material - with
a design scope in regard to speci~ic heat capacity of the
order of 1:2. ~his range can be further broadened by
corresponding geometric configuration (different wall
thicknesses) and the incorporation of porosities.
With increasing specific heat capacity of the
hydrocarbon adsorber compared with that of the following
three-way catalyst, heating up is delayed in relation to that
of the catalyst. Accordingly, the adsorber retains its
adsorption capacity for a longer period and only releases the
adsorbed hydrocarbons after a certain time. If the ratio of
the specific heat capacity of the hydrocarbon adsorber to
that of the three-way catalyst is selected from values of 1.1
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to 3.0:1 and preferably from values of 1.5 to 3.0:1, the
effect of the delayed heating of the adsorber is that
desorption of the hydrocarbons only begins when the following
catalyst has almost reached its full effectiveness in regard
to the conversion of hydrocarbons. For example, if the
hydrocarbon adsorber has double the specific heat capacity of
the following catalyst, the adsorber heats up only half as
quickly as the catalyst for substantially the same energy
input.
One particularly favorable embodiment of the process
according to the invention is characterized in that the
hydrocarbon adsorber and the three-way catalyst are
monolithic honeycomb supports of which the heat capacities
are 1.10 to 3.0:1 and to which hydrocarbon-adsorbing coatings
or catalytically active coatings are applied in known manner.
A temperature-stable dealuminized Y-zeolite with an Si
to Al ratio of greater than 50 and preferably greater than
100 is preferably used as the hydrocarbon adsorbex, being
applied in a quantity of 100 to 400 g per liter honeycomb
volume. A Y-zeolite such as this shows high temperature
stability and does not lose its adsorption properties, even
after repeated heating to around 1000~C - the operating
temperature to be expected in the vicinity of the engine. In
addition, an adsorber of this type shows selective adsorption
behavior for hydrocarbons, i.e. it adsorbs hydrocarbons
preferentially to the steam also present in the exhaust gas.
The emission of hydrocarbons during the cold start phase
can be further reduced if the adsorber itself shows catalytic
properties. These can be achieved by providing the coating
of hydrocarbon adsorber with additional amounts of a typical
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catalytically active coating. Typical catalytically active
coatings usually contain large-surface carrier oxides, such
as for example lattice-stabilized or pure aluminium oxide of
the transition series, doped or pure cerium oxide and doped
or pure zirconium oxide. Catalytically active metal
components from the group of platinum metals are applied to
these carrier oxides. The ratio by weight between zeolite
adsorber and carrier oxides in the coating should be from 4:1
to 1:2. The catalytically
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active components from the group of platinum metals
should be finely distributed over all oxidic parts of
the coating except for the zeolite adsorber. Preferred
platinum metals are platinum, palladium and rhodium.
A coating of the type in question may be obtained
by initially preparing a coating dispersion of zeolite,
aluminium oxide, cerium oxide and zirconium oxide to
which the catalytically active metal components are
added in the form of their precursors, such as for
example nitrates or chlorides. It is known that these
precursors are deposited preferentially onto aluminium
oxide, cerium oxide and zirconium oxide, but not onto
zeolite. The monolithic support is coated with this
dispersion by methods known per se, dried, calcined and
optionally activated in a hydrogen-containing gas stream
at temperatures around 600CC.
Even better separation of the noble metal com-
ponents from the zeolites is obtained if a mixture of
aluminium oxide, cerium oxide and zirconium oxide is
first precoated with the noble metals in a separate
impregnat~ng step and the final coating dispersion of
zeolite and the other oxide components is only prepared
after the noble metals have been fixed on those com-
ponents by calcination.
2S In one particularly preferred embodiment of the
invention, the support is coated with two different
layers. The first layer is a catalyst coating of large-
surface carrier oxides and catalytically active metal
components from the group of platinum metals. The
actual hydrocarbon adsorber coating is then applied to
that coating. The quantities of coating to be applied
range from 50 to 200 g per liter support volume for both
layers.
Ceramic monoliths of cordierite or mullite are
preferably used as supports for the hydrocarbon adsorber
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and the three-way catalyst. Other suitable materials for the
supports are zirconium mullite, alpha-aluminium oxide,
sillimanite, magnesium silicates, petalite, spodumene,
aluminium silicates, etc. or even stainless steel. The
ratios between the specific heat capacities of these supports
must correspond to 1.10 to 3.0:1. The data of a few
commercially obtainable ceramic monoliths are set out in
Table 2. The monoliths in question are honeycombs with a
diameter of 93 mm and a length of 152.4 mm, with various wall
thicknesses and with a cell interval of approximately 1.28
mm.
Table 2
Monolith Wall Specific Weight Heat
thickness heat capacity
tmm] [J g~l K~l] tg] [J K 1]
A 0.16 0.850 351 299
B 0.16 0.845 440 372
c 0.16 0.843 471 397
D 0.14 0.862 427 368
E 0.16 0.836 448 375
F 0.19 0.824 581 479
G 0.25 0.836 706 590
In another embodiment of the process according to the
invention, the support for the hydrocarbon adsorber is a
ceramic monolith while the support for the catalyst is a
heatable metal monolith. The ratio between the heat
capacities of the hydrocarbon adsorber and the three-way
catalyst must again satisfy the conditions of 1.10 to 3.0:1.
The hydrocarbon emissions during the cold start phase can be
further reduced in this way because the delayed desorption of
the hydrocarbons is combined with accelerated heating of the
catalyst.
The invention is further illustrated by the following
Examples.
Example 1
The adsorption and desorption properties of an adsorber
coating on various ceramic honeycombs of cordierite were
compared with one another. The adsorber coating consisted of
dealuminized Y-zeolite with an Si to Al ratio above 100.
This coating was applied in a quantity of 100 g/l honeycomb.
The adsorbers had a cell density of 68 cellsjcm2 , a diameter
of 25.4 mm and a length of 152.4 mm. A total of three
different ceramic honeycombs (A, C, G according to the above
Table) with mass ratios of 1:1.34:1.98 and corresponding
ratios between their specific heat capacities were used.
Testing was carried out in the model gas under the
following conditions:
- Gas mixture before the adsorber 200 ppm
toluene, rest N2
- Temperature heating of the gas mixture from
So to 200~C at 10 K/min.
- Gas flow rate 1550 Nl/h
- Measured quantities temperature before and after
the adsober; toluene concen-
tration after the adsorber
The key results are set out in Table 3 which shows the
quantities of toluene in percent released by the particular
adsorber at the temperatures indicated, based on the
quantities of toluene desorbed from the lightest adsorber at
those temperatures.
Table 3
Monolith Temperature before the adsorber
120~C150~C 180~C 200~C
A 100%100% 100% 100%
C 53%95% 100% 100%
G 53% 83% 91~ 92%
During the test, a total of only 53~ of the quantity of
toluene released by the monolith A up to a temperature of
120-C is found, for example, behind the monoliths C and G up
to the temperature of 120~C. In conjunction with a following
catalyst having a smaller specific heat capacity, this
delayed release leads to a considerably improved hydrocarbon
ConVersion in the cold start phase.
Example 2
The heating-up behavior of monoliths C and G was studied
in a second test. To this end, air heated to 400~C was
passed through the monoliths under flow conditions typical of
3~ automotive emission control catalysts. The increase in
temperature as a ~unction ~f time at the exit end of the
monoliths was measured by thermocouples. The results
obtained are shown in Table 4 where the time taken to reach a
certain temperature is shown in seconds. The slower heating-
up behavior of the heavier monolith G can clearly be seen andleads to an adequate delay in its desorption behavior where
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it is combined with a following catalyst on a ceramic
monolith of the A type.
Table 4
Temperature after the adsorber
MonolithlOO~C 150~C 200~C 250~C
C 18s 25s 32s 40s
G 25s 34s 42s 55s
