Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2 ~
The invention relates to an exhaust gas purification system
for reducing hydrocarbon emi~sions during the cold start of
combusti~n engines. The exhaust gas purification system
contains a hydrocarbon adsorber and a downstream catalyst
system, which may comprise one individual three-way cataly~t
or a combination of oxidation, reduction, and/or three-way
catalysts in one or more beds.
The future limit~ for pollutant emissio~s from motor vehicles
are laid down in the regulations Tn~V/1994 and L~V/1997 (LEV =
Low Emission Vehicle~. They represent a substantial
tightening of the limits, particularly for hydrocarbons. As
present-day exhau~t ga~ catalysts have reached a high level of
pollutant conversion in the hot operating state, it is
possible to comply with the future limits only by an
improvement in pollutant conversion during the cold start
phase. This is because a large proportion of the hydrocarbons
released as a whole is emitted during the cold start phase of
the test cycles laid down by law (e.g., US FTP 7~. In said
phase, the catalysts have not yet reached the operating
temperature of 300 to 400~C required for conversion.
The hydrocarbons emitted during the cold start phase are
chiefly Cl to C10 compounds such as paraffins, isoparaffins,
olefins and aromatics.
,: .
In order to reduce the pollutant emissions during the cold
start phase, an exhaust gas purification system comprising a
hydrocarbon adsorber and a downstream catalyst is proposed,
for ~xample, in US Patent No. 5,078,979. The hydrocarbon
adsorber has the task of adsorbing the hydrocarbons contained
in the exhaust gas during the cold start phase at temperatures
that are still relatively low. Only when the adsorber becomes
hotter are the hydrocarbons desorbed again and arrive with the
now hotter exhaust gas at the catalyst which is now almost at
-- 1 --
2~0~
operating temperature and are convexted here to harmless water
and carbon dioxide. An important requirement in respect of
the adsorber is the abili~y to adsorb hydrocarbons
preferentially before the water vapor which is al50 present in
abl~n~nce in the exhaust gas.
A disadvantage of said described solution is the desorption of
hydrocarbons commencing ~ven at relatively luw temperatures,
with the result that opti~um conversion on the downstream
catalyst cannot yet take place. Usually, there is a
temperature gulf of m~re than 100~C between the light-off
temperature TA ~f the catalyst of 300 to 400~C and the
desorption temperature TD ~f the adsorber ; -~iately ~ eam
of abouk 150 to 200~C, i.e., T~ - TD > lOO~C. Moreover, there
is the risk of thermal destruction o~ the adsorber since it
has to be incorporated near the engine in the exhaust gas
purification system and is therefore exposed to temperature
loads o~ up to 1000~C during continuous operation.
In order to overcome 6aid disadvantages, there is a large
nu~ber of proposals in the patent literature, for example, in
German Specification DE 40 08 789, in European published
Patent application EP 0 460 542, and in US Patent No.
5,051,244, Sai~ documents likewise start out Prom
~5 the combination of a hydrocarbon adsorber and a catalyst, but
propose complex circuits for the exhaust gas in order to
overcome the di~advantages descrlbed.
For example, USP No~ 5,051,244 proposes to provide a molecular
sieve adsorber upstream of the actual catalyst, which in the
cold state adsorbs the pollutants in the exhaust gas,
- 2 ~
particularly hydroc~rhor-~, and releal3e~ them again as th~
~h~ t gas purification system becs~mes hs:~'cter.. In order
to protect the ad~;orber from destruction by overheati~g
5 wheI~ the erlgine is ill CO~lti~UOU~; operation, provision is
made for a hort s::irl::uit line which c:an be c:o~nected ~rom
the ~n~!Ji n~ directly to the cataly~;t.
Duri~g the fir~;t 200 to 300 seCOn~l~ after th}~ etar$, the
10 ~h~ t gas is ~ se~ omple1:ely~ver the ~ orher and the
catalyst. II1 said operating phaf;e, the hydro~rhor~: ~re
A~ rhe~l by th2 ~orh~r. A~$orber aIld cataly~;t are heated
to aD~ i~crea~ing e~teIlt by the hot ~h~ t ga O The
~orher i~ short-circuited if de~i:orption heg; n~ to e:cceed
ads~ ion a~ a result of the tempsL~Le i~crea~e. The
~hA~l~t gas now flows directly over the catalyst. Whe~ the .
: .opPrsting tempela~u~e is reA~he~, part of the hot exhau~t
ga~ i~ pa~e~ over the adsor~er u~til complete desorption
of the pollutants, which may ~ow be converted by the
catalyst with a good level of e~ficie~cy. After desorptio~
:has take~ place, the ad~orber i~ sh~rt-circuited ag~i~ i~
'. order to protec~ it from de~truc~io~ by thermal overloadO
,
Ad~orber~ proposed by USP 5,Q51,244 and U~P 5,078,979 are
25 :natural or syn~hetic zeoli~es with an Si/Al atomic ratio of
at least 2.4. Suitable zeolites me~io~ed are Qilicalite,
faujasite, clinopkilolite, morde~ite, chAhi7.;te,
iultra~table Y-zeolite, Y-zeolite and ZSM-5 and mixtures
thereof. The zeolite adsorber may, moreover, contain
finely divided, catalytically active metals ~uch as
platinum, palladium, rhodium, rutheni~m, ~d mi~tures
thereof.
Said solutio~s known from the state of the art are either
technically very complex~ expensive a~d ~u~ceptible to
breakdown or, as in ~he case of USP 5,078 t 979, lack a
~olution for ~ridging the temperature gulf between the
,,
- - 3
2.~
desorption temperature of the adsorber and the light-off
temperature of the downstream catalyst.
The invention provides an exhaust gas-purification system ~or
reducing hydrocarbon emissions during the cold start of
combustion engines containing a hydrocarbon adsorber and a
downstream catalyst system composed of a three-way catalyst
or a co~bination of oxidation, reduction, and/or three-way
catalysts in one or more beds. The exhaust gas purification
~,ystem is characterized in that the dif~erence between the
light-off temperature T~ of the oxidation or three-way
catalyst for the conversion of hydrocarbons and the
desorption temperature TD ~f the adsorber ; -~;ately
upstxeam is less than 50~C, i.e.,
TA ~ TD < 50~C.
The light-off temperature TA ~f ths catalyst means in this
case the exhaust gas temperature before the catalyst at which
the catalyst conYerts exactly 50% of the hydrocarbons.
~0
The desorption temp~rature TD of the adsorber is a parameter
which may be determin~d from the engine only in dynamic
operation. To this end, the crude hydrocarbon emission of
the engine without the use o~ an adsorber is recorded as a
~5 function of time initially during the ~irst 200 to 300
~econ~R after the cold ~,tart. Said crude emission typically
shows a high and broad ~; during th~ first 60 to 100
seconds. As the engine bec~ ~s hotter, the hydrocarbon
emission falls to the normal level when the engine is hot.
In a second test run, the hydrocarbon emission after the
connected adsorber and the temperature before the adsorber is
then a~ red as a function of time.
- 4 -
~ ~ .
As a result o~ the adsorber, the hydrocarbon emission is
initially greatly suppressed by adsorption, ~ut t~en
increa es as the exhaust gas becomes hotter a~ ~ re~ult o~
increasing desorption by the adsorbPr and likewise passes
through an emi~sion maxi~um with a time delay 50mrAred w~th
the crude e~ission, be~ore it eventually falls to the value
of the crude emission when the engine is hot. A5 a result
of ~he time 5hift in ~he ~mission maxi~a of the crude
emission snd ~he emission with adsorber, the two emission
curves int~rsec* at a particular time wi~hin about 60 to
100 seco~ a~ter the cold staxt.
The exhaust gas temperature present before the adsorber at
th~.s point in tI~e is known as the desorption t~ ~eYatUre
15 TD of the adsorber~ It depends on the design o~ the
exhaust gas ~ystem in each case and on the adsorber
material itse~f, and is typically ~etween 150 and 200 ~C.
Zeolites are used as prePerred ~dsorber materialc. As
already disclos d in USP 5,051,244, however, only those
zeolites that adsorb hy~ocarbons preferentially before
water, i.e~, which are ~yd.o~hobic and, moreover, have a
high t~ L~e and acid stability, are suitable for the
adsorption ffl h~,ocarhons from the exhaust gas of
25 combustion engines.
The hy~locarbon ad~orber should contain at least one
hy~lo~hobic, temperature~ and acid~stable zeolite with an
Si/Al ratio of more than 20. In a particularly favourable
embodiment of th~ in~ention, two zeolites which have
unequally steep temperature curves of their hydrocarbon
ad~orption capacities are combined with one another in the
adsorber. At least two zeolites I and II should be
combined, of which zeolite I has a greater adsorption
capacity at temperatures below 100 ~C than zeolite XI, and
zeolite II has a greater adsorption capacity above 100 ~C
than zeolite I. The zeolite I used is, for example, a
- 5 -
- ,. , .. . . . .; ; ~ " :: .
2 ~ L ~) 3.. ?J ',,1
deall i ni F~d Y-zeolilte with an Si/~l ratio of more thall 40
~d the zeolite II used iE; a zeolite Z5M5 with an SifAl
ratio of more tha~ 20 . iDealt~; ni ~e~ Y-zeolite and zeolite
ZS~5 ~:hould be pre~n'c in the adsorber in a weight rat1o to
5 one another of 1:10 to 10.,1.
- ZQolite Y belong~ to the wid~-pore zeoli~es wi~h a pore
diam~ter of 0<.74 nm, a pore volume of 0.3 ml/g ar~d a
~;pec~fi~ surface area of more than 700 ~112/g. Zeolite ~S~q5
10 i~ a medium-pore 2eoli~ce with a pore diameter of a~,o~,.
O 0 55 nm. A~ a result of it8 large pore aperture, the
~-zeolite has a high i~itial adsorption capacity for the
aromatics cont~; ne~3 i~ the ~h~ t ya~;. The adsorptio~
c~r~c; ty fall~ very rapidly, hcu~ve., as the tempe~c-LllLe
lS ril3es. Zeolite ZSMS, on the other haDd, ha~: a lower
i~tial ad~orptiol~ c~rA~;ty for aro~nati¢~, but e~h~hits a
~;maller fall in ~aid ~ i ty as the tempe~ ,u~e ri~;e~.
Moreover, said zeolite ha~; a good adsorption capacity for
other h~ LlL~c2rhons still coIltained iIl the e~hau~t gas ~ The
20 combinatio~ of the two zeolites according to the inve~tion
leads to an opti~um adsorption behaviour in the tempel~L~le
rallge conc~rn~ he invention i8 1~lt)1:, howe-ver~ ~onfined
to a mi~ture only of ~;aid tws:~ zeolite~ Other zeolite
ture~ ~ay al~o be u~;ed i~E their co~nponents ful:Eil lth~
,~ 25 requirementa; regarding tempeL~u~e dep~n~enc:e of the
ad~orpltion capacity and pore diameters.
The high Si~Al ratio of the zeoli~es to be u~ed according
to tl~e invention guarantee~, ODL the oIle hand, high
30 ~electivity o:f lthe adsorption of ky~lL-~carbo~s compared with
water and, on the other hand, good tempeLd~le ~hi 1 ;ty t:o
1000 ~C and above, and ~food acid ~hi l; ty. The
temperature ~t~h; 3 ;ty i2; ~ece~:~;ary iEor the e~a~ t ga
puriEication sy~'cem according to the invention siIlce the
; 35 adsorber i~ placed ~ear the Pn~ine and i~ thu~ expo~ed to
high temperatures in operation.
''
~ - 6 -
.
~ . , : : .
2 ~ 3 3~ ~ ? ?
The cataly~t system dnwnstream of the hyclLrocArhorl adsor~er
may ~ompri~;e a three ~y catalyst or a r~ ~i n;~tioll of
o:~idatioll, reduction and/or three-way caLtalyst~ in one or
more l~edsO
Such cataly~;ts arld th-3 preparation thexeo:lE are kIlown to the
e~pert. They usually compri~e a ~ o~ the form of an
opeal-cell honeycomb body made of ceramic or metal., In
order to accept cataly~i c~l ly active noble metals, said
10 honeyco~ bOC~i<?fi are prc~vided with anL activi1ty-i~cre~;n~,
high ~;urface area o~ide ~ p'~r~;ion coating ~made of, for
e~:ample, y-aLl~nium o:Kide i~ a quantity of laO to 400 g,
nAlly 160 g per litre of honeycomb body ~o~ume. The
catalytically active noble me1;als may be ~epo~ited o~ ~aid
15 o:cide coatillg by impregnation.. In the case of o:Kidatio~
cataly~t~, p~ and/or p~ ; um are used -i~
prefere~ce. Three-way catalysts co~tain platiIlum and~or
p~ d/or rhodium as ca~al~cA11y actiYe llobl~3
met:als.
In an ~P~h~ t gas purificatio3~l sy~;tem according to the
inventio~ composed of adsorber, o:~idation ca~aly. * a~d
threc u_y e:ataly~;t, the l~ i n~ of the oxidation ~ataly~:t
with platinum and~or pA11A~ium compared wi~h the loadi~g of
25 . co~yentioIlal o~idation catalysts of 0.Ol to l.8 g per li~cre
of catalyst volLune i~ at . least doubled to at least 3 . 5 g of
platiIlum aIld~or p~ ; um per litre of cat~ly~t volume. In
pref erence, the loading ~;hould be 7 g per litre or more .
T.o~Ain~~ with more than lO, or more than 20 g of nobl~
30 metal per litre of catalyst volume are particularly
ef~ective. The oxidation catalyst is placed immediately
after the adsorber. Said high loadirlg with the
catalytically ac~iv~ elements leads to a reduction in the
light-off tempeLcli.u,e by about 50 to 100 ~C compared with
35 Ilormally loaded cataly~ts.
If the catalyst system downstream of the adsorber i~ compo~ed
only of a three-way catalyst with the platinum group metals
platinum and/or palladium and~or rhodium, the loading with
platinum and/or palladium compared with the loading of
conventional catalysts of 0.01 to 1~8 y per liter of catalyst
volume can also be at least doubled with said catalyst to at
least 3.5 g of platinum and/or palladium per liter of cataly~t
volume in order to reduce the light-off temperature for the
conversion of hydrocarbons.
The adsorber may be used as bulk material in the form of
pellets, extruded pieces or agglomerates. The use of the
adsorber in the form of a dispersion coating on a monolithic
honeycomb body in a quantity of 100 to 400 g per liter of
honeycomb body isl howeYer, preferred. The actual quantity of
coating to be used depends on the hydrocarbon emissions of the
combustion engine to be detoxified. The optimum quantity may
be determined by any expert with few tests.
The dispersion coatinq is deposited on the honeycomb body, for
e~ample, b~ immersing the honeycomb body in an aqueous
dispersion of the adsorber mixture followed by blowing out
~ce~s dispersion, drying and, if necessary, calcining to fix
the coating. In order to apply the desired quantity o~
ad~orber, said coating may be repeated many times, if
necessary.
., .
A further possibility consists in an exhaust gas purification
system for reducing hydrocarbon emissions during the cold
start of combu~tion engines, which contains a hydrocarbon
adsorber in ; -~iate contact with an oxidation c~talyst, and
a downstream three-way cataly~t in one or more beds. The
exhaust gas purification system i6 characterizsd in that the
difference between the light-off temperature T~ of the
oxidation cataly~t for the conversion of the hydrocarbons and
. - 8 -
.~
~i ~ ' , i , ,
2 ~
the desorption temperaturP TD ~f khe adsor~er in immediate
contact with the oxidation ca~alyst is less than 50~C, i.e.,
TA - T~ < 50~C.
5 The ; -~iate contact of the adsorber wi~h the oxidation
catalyst may be realized in the form of superimposed coatings
on a monolithic honeycomh body, the adsorber coaking lying on
the catalyst coating.
The statement already mad~ above applies to the choice and
design of adsorber mixture and catalyst. Apart from reduced
hy~Loccarbon emission, said exhaust gas purification systems
also exhibit a substantially-reduced carbon monoxide emission
during the cold start phase.
The invention will now be explained in more detail on the
basis of some examples and with reference to the accompanying
drawings in which:
Figure 1: Hydrocarbon emission of a combustion engine
with exhaust gas purification system according to comparative
example 3a during the cold start phase of the US FTP-75 test.
Figure la: Diagrammatic representation of the structure of
the exhaust gas purification system.
Figure 2: Hydrocarbon emission of a combustion engine
with exhaust gas purification system according to comparative
example 3b during the cold start phasie of the US FTP-75 test.
; Figure 2a: Diagrammatic representation of the structure of
the exhaust gas purification system.
Figure 3: Hydrocarbon emission o~ a combustion engine
with exhaust gas purification system accordin~
:'
... :
to example 3 during the cold start phase of the US FTP-75
te~t .
Figure 3a: Diagrammatic representation of the structuxe
5 of the exhaust ga~ purific:ation ~ystem.
~ga~nple 1 ~d~orp~ioa proper~ie~ o~E zeoli~Q ~ and ~SM5
The ad~orption capacity of a iD~Y zeolite ~deal~i~;ed Y-
10 zeolite3 with an 8i/al ratio o~E ~100, and of two zeoli1;e~
ZS~[5 with the Si~Al ratio~ of >500 and 58 wa~ deter~i~ed
for toluene ~t 20 alad 80 ~C. The results are ~;hown in
Table 1.
15 Table 1 Ad~;orptio~ c~rz~c; ty of a D~Y zeolite a~d of ZS~5
Tt~C~ Zeolite Si/Al ~Toluene Lg/100 g]
D~ >100 15.1
ZSM5>500 6 . 2
20 20 ZS~q55~ 7~ 1
DAY~100 0 . 8
ZS~5>500 1 . 4
~0 Z~:~558 2 . 2
25 The data in Table 1 are applicable to a toluene
conce~ ~a~iOn of 1 gjm3 air. ~oluene i8 the amount of
~ad~orbed tolue~e which i8 in eql~; l; hrillml with the
~urrounding a; ~phPre at the temperature given in each
c~se, in granunes of. toluene per 100 g of ~eolite. Table 1
30 showE; very clearly the different ad~;orption behaviour of
the DAY zeolite a~d of zeolite ZSM5. Whil~;lt the DAY
zeolite e~h; hi t~; a~ ea:cellent ad~;orptio~n c~ ci ty for
tol~ene at low temp~~ra*ured which falls very ~uiclcly,
hc~wever, as the t~mpeLaLu~e ri~;es, the correspondiIlg c:urve
35 for the ZSN5 zeolite i~3 much flatter. EveIl at 80 3Cr
zeolite ZSM5 is ~uperior to the DP.Y eolite~ A mi~ture of
-- 10 --
. S' .~ ~ ~ !. ' ' , .
2 ~ 3 ~~)
both zeolites give~3 ~ more lmiform ad~orption behaviour
over a f airly large temp~rature rallge .
E~ampl~ 2
The light-off tempela~llLe TA :EOr ~he conYer~ n of
hydroc~r~n~ by pAl~Aium o:~idaltion ca~alyst~ with
differi~g p~ tlium lo~l;ng and of a 8t~nrl~rd
pl~ti~um/rhodium thrc~ cataly~;t in the iEre h and aged
10 stalte at space velocities o~ 75,000 h~l and 60,000 h 1 a~d
with an air ratio l. ~ of 1.15 were mea~;ured.
The cataly~;t2; were c ,. ~d of the relevant oxide
p~r~;ion coaking of 160 g y alu~inium c~ id~ per litre on
15 ceramic hon~ ' bodies made of cordi~rite and the
catalytically active noble met~l~ preripitated thereon.
The honeyc ' bodies had a cell densilty of 62 CQ11S per cm2.
~rhe light-ofiE tempe~a~u- es are listed i~ Table 2.
Table 2 ~ight-off tempe~ .e~ of various catalyst~;
lamhda = 1.15
; T~i n~ TA t~C] Space velocity
tg Pd~l~ Fresh Aged [h-1J
y 25 3.53 226 237 ~,000
~ 5.30 227 232 75,000
; 7.06 220 ~35 75,000
: 10~59 219 220 75,000
lg1 Z09 60,000
30 40 189 204 60,000
S Pt/l Rh standard three-wa~ catalyst
.41 g/l 251 286 ~,000
I~ order t~ measure the light-off temperatures of the aged
- cataly~ts, they were operated for a period of lOo hours on
the engine at e~haust gas temperatures before the catalyst
,. -- 11 --
''
, 2~.a~
of 850 ~C. I~ view of ~he heat of reaction, this leads to
tempeLaL~lLes in the c:ataly~~ bed of lûO0 ~C~
The pal ~ o~idation ca'calystE; of Table 2 have a ~uch
5 lower light-off temperature for ~he conver~ion of
hydro~Ar~ than the sta~dard threc ~._y cataly~t. The
high :~g~ing~ s$abili~ of their light off tempe~L~lL~, which
can be attributed to the high p~llA~ m loading, i8 al~;o
worth ~oting. Table 2 also E~hows that the lig~lt o:lEf
ïO tempel~l ~e of the p~ o~idatioII cataly~;t fall~;
co~;iderably with incre;~in~ palladium lo;~l;ng.
The lighlt~off tempeL&~ s of ~aid highly loaded p~
o:Ecidaitio~ ca~aly~ 3, with values of below 237 ~C, lie jUf;t
15 above the typical desorption te~pe~ aLIl~es of the adsorber~:
and ~hi~it light-off tempe~ u~es at ~bout 2.00 ~C with
very high lo~li ny5~. ~hey are capable of converting
directly the hydro~;~rhon~: de~;orbing from t;he adsorber at
al~out 200 oc without the comple~ ~h;~ t gas circuits known
20 from the ~tate of ~he art. The sta~dard three-way s:atalyst
i~ not able to do so because of its high light-of î
tempeldl.~ , particularly in the aged ~3tate.
The l~ cz~rhon em;~ of a motor vehic:le with a~ Otto
25 motor ~Mercede~; 300 E; c~r~ci~y: 3 l, power: 16~ kW~ were
meaE;ured below during the cold ~tart: pha~e for di:Eferent
Q~hAl-~t gas purification s:ystems in accordance with
e~camples 3 and the co~nparative egamples 3a a~d 3h. The
re~ults of the resi ~ i s~; on measurement~ accordiIlg to :
30 the US FTP-75 tes~ are summarised i~ table 3.
The e~rhAll~t gas purification sy~tams comprised in each case
3 susces~ive ho~eycomb bodies made of cordierilte with 62
cell~ per cm~. The honeys~omb body Oll the en~; ne ~ide had a
35 length of 154 mm and a volume of 1. 8 l.. The two xear
honeycvmb bodies each had a length o:E 102 mm and a volume
o~ 1.2 l.
-- 12 --
~! .
3 ~
Said honeycomb bodie~3 wer~! coated as follow~ for compari~on
of all e~chau~t gas purification ~3y~tem acc~srding to the
inventioIl with conven~;on~ tems:
5 ComparatilJe e~ample 3a ( Fig . 19 la
1.-3 ~o~eycoDlb body: Coati~g with a tandard thrc:e w_~
catalyst according to e~ample 2;
aged
Co~parative e:~a~ple 3b ( Fig . 2, 2a j
1. ~oneyco~b body: ~oa~ing with 100 g/l OI DAY
lS zeolite ( Si/Al > 100 )
2~-3 Holl~yc~ ' body: Coati~g with a standard three w...... y
cataly~;t according to e:~:ample 2;
aged
E~cample 3 ( Fig . 3, 3a )
1. Honeycomb body: ~rhe first honeycolub body was
replaced by two partial bod:Les.
~he partial body 52 mm long on the
engine 8id~3 xeceived a coating oiE
100 g/l of DAY zeolit~ ( Si/Al >
100~. The second partial body was
. coated with an ox.idation catalyst
with 7 g of !Pd per litre of
honeycomb body volu}ne. These
coatîng~:, too, were aged befoxe
the e~:haust gas tests were carried
out .
. ,
-- 13 --
3 ~
2.-3 Honeycomb ibody. Coatirlg with a ~;talldard three-way
cataly~;t acc:ording to e~ample ~;
aged.
5 ~i~res 1 to 3 ~how the e20is~ioIl curve~ for hydroc~r7~n~:
~3ur;9~g the fir~t 250 ~ af~er the ~3tart when variou~; e~hau~;t
ga~; purifica~io~ ~;y~;tems .re used (Figalre~ la - 3a). The
give~ hydrocarl~o~ collce~tratio~3 re~ate to a~ h~ t gas
diluted to one terl~h by air in accordance with the l~S FTP-
10 75 test spec; fi cation.
Figure 1 hows that ~he aged thrc::e u..y c~taly~:t~: ( accordingto Figure la~ begin after about 50 tq~Con~ to collv~:l L the
pollutants i~ the ,p~rhF~ t ga . At thi~ pOillt in time, the
15 tempeL&~ e i~ the e~h~n~t ga ~efore the cataly t8
300 ~C. Figure 2 ~hows the ~ame rela~-; on~h; p8 as Figure 1,
but with an A~or~r wi~h a D~Y zeolite ups$ream of the
throe ~.~y cataly~t i~ the ~A~ct gas purification ~ystem
(Fig. 2a~ The desorptio~ of h~dro~rhon~ ~y the adsorber
~egin~ after only abou~ 30 ~e~o~ t a ~empe~Lu~a of
about 200 oc i~ front of the ~or~r. The three-way
cataly8t8 are ~ot yetO however, capable of co~velLing the
majority o~ the ~e~orhin~ ~y~carbon~. Figure 3, in
co~a~t, ~hows a - he~ decrea~e i~ the re~ l emis~io~
25 :as a result o~ co~bi~i~g the adsorber with an o~idation
cataly~t highly loaded with palladium in co~junction with
the two sta~dard three-way catalysts (Fig. 3a). The
h~tched area in Figure 3 represents the reduction in
~ hydrocarbon ~ iSs;on of the exhau~t gas purification system
- 30 according to the inventio~ according to e~ample 3 in
- comparison with the conventional ~h~ t gas purification
system according to comparati~e example 3b~
- The re~ ;a~ion mea~urements o~ the e~h~ t gas
.~ 35 puri~icat on ~ystems according to comparative e~ample 3b
a~d accordi~g $o e~ample 3 are shown in T~ble 3. As these
measurements ~how, the use of a highly loaded palladium
., 1
2 ~ ?~ ~
oxidation cataly~t i~ combination with hydrocarbo~ ad~orbi3r
and ~andard ~hree-way catalyst~ has a po~3itive eff2ct on
the residual ~ i oal~; of the e~haust gas 5y tem. During
- the cold ~;tart p~a~;e, not only ~r~3 the hydroc:ar~
5 ~ olls reduced by 3û%, but al~;o ~he ~ ;on o~ carbo~
?no~i~le. Said po~;itive effect al~o l~ ~in~: throughout the
erltire test.
Table 3 Re~:idual e~i ~:~;; 9n mea~;urement according to
~S FTP-75
~h;~ t ga~; ~;y~;tem Conte~t of the fir~;t bag i~ g~miles
~ Cold start pha~e )
CO HC NOX
15according to
comparative e~:a~ple 3b* ~ 3 . 00 0 . 51 0 . 83
ac~ording to e~:ample 3 2.05 0.37 0.89
~:~hA~ ga~ ~y~;tem Total ~ i on in g/mile~:
C~ ~IC ~~X
accordi~g to
,c~ ~ =rative e:f~ample 3b 1. 29 0 . 22 0 . 46
according to e~ample 3 0.48 0.10 0.38
* 3 = Compaxative e:Eample
-- 15 --
