Process and catalyst for the reduction of the ignition temperature of diesel soot filtered out of the exhaust gas of diesel engines

A catalyst process is disclosed for the reduction of the ignition temperature of Diesel soot filtered out of the exhaust gas of Diesel engines by passing the exhaust gas over a catalytically active substance, which is selected from lithium oxide, copper chloride, vanadium oxide/alkali metal oxide combinations, a vanadate of an alkali metal or of cerium, or a silver or alkali metal perrhenate, preferably of sodium or silver, or mixtures of these substances.

The invention relates to a catalyst and process for the reduction of the 
ignition temperature of Diesel soot filtered out of the exhaust gas of 
Diesel engines. The invention is a further development in the treatment of 
Diesel exhausts as described in our copending U.S. patent application Ser. 
No. 434,806 filed Oct. 18, 1982, now U.S. Pat. No. 4,455,393, the entire 
disclosure of which is incorporated and relied on herein. 
Diesel engines, because of their method of operation, emit soot particles 
or very fine droplets of condensate or a conglomerate of the two 
("particulates"), in addition to the usual harmful substances, such as 
hydrocarbons, nitrogen oxides and CO, as emitted also by gasoline engines. 
These "particulates", referred to herein as "Diesel soot" are particularly 
rich in condensed, polynuclear hydrocarbons, a few of which have been 
found to be carcinogenic. 
The proposal has previously been made to capture the soot and condensate 
particles in traps or filters. Since, however, the temperatures of Diesel 
exhaust gases under normal operational conditions are not sufficient for 
burning off the accumulated soot, as for this at least 
450.degree.-600.degree. C. is needed depending on the engine related 
composition of the soot, care must be taken for a timely increase of the 
exhaust gas temperature for the regeneration of the trap or of the filter, 
in order to avoid an accumulation of soot and thus of obstructions. This 
may be accomplished, inter alia, in such a way that the air/fuel mixture 
formed in the engine is periodically enriched and thus higher exhaust gas 
temperatures are produced. Another possibility provides for the 
disposition of a burner in the exhaust gas system before the filter, which 
is ignited as needed and takes care of the temperature required for 
burning off the soot. 
These methods of solving the problem are, however, connected with 
additional fuel consumption and thus partially reduce an important 
advantage of the Diesel engine. 
It has now been found that the ignition temperature of Diesel soot may be 
lowered by equipping the soot filter or the soot trap with a special 
catalyst or with a temperature stable substance containing it, and that 
thereby it is possible to achieve especially an important decrease in fuel 
consumption during the regeneration phase of a filter. 
The present invention is a further development of the invention disclosed 
in our above mentioned U.S. patent application which relates to a way for 
lowering the ignition temperature of Diesel soot filtered out of the 
exhaust gas of Diesel engines, with the use of silver vanadate as 
catalytically active substance. 
Consequently, the object of the invention is a process for reducing the 
ignition temperature of Diesel soot filtered out of the exhaust gas of 
Diesel engines which is characterized in that the exhaust gas is conducted 
over one of the following catalytically active substances: 
lithium oxide, 
copper (I)-chloride, 
vanadium pentoxide with 1-30% by weight of alkali metal oxide, 
a vanadate of cerium or an alkali metal preferably lithium, sodium, 
potassium, or a perrhenate, preferably of silver or an alkali metal such 
as sodium or potassium or 
a combination or mixture of two or more of these substances. 
The catalytically active substance of the invention is used in a 
catalytically effective amount and may be precipitated on a thermally 
stable (temperature resistant) carrier substance or be mixed with it. As 
carrier materials, conventional substances, especially transitional 
alumina may be used, other examples are silica, titanium dioxide, 
zirconium oxide and oxides of the rare earth metals, as well as copper 
chromium oxide (CuCr.sub.2 O.sub.4), nickel oxide and iron oxides. 
It is preferable that the catalytically active substance or its combination 
with a carrier material is applied to a filter element serving as 
structural reinforcer. 
As a structural reinforcer, a packing of temperature resistant, shaped 
metal body or mineral wool, or a filter element according to German OS No. 
29 44 841 or German OS No. 29 51 316 or preferably a monolithic ceramic 
body with numerous flow channels may be used, the channel openings of 
which are provided on opposite end surfaces with closing means in such a 
way that a channel open in one front surface is always closed in the 
opposite end surface. Such catalytically inert carrier bodies are well 
known in the art. 
An effective development for practical use of the process of the invention 
provides that the filter element is disposed in the flow cross-section of 
a housing provided with inflow and outflow channels for the exhaust gas; 
i.e. in the exhaust gas stream. 
A desirable variation of the invention provides that there be a noble metal 
impregnation in addition to the active substance. It is preferred that the 
active substance is applied in the area of the filter inlet side and the 
noble metal impregnation subsequently thereto in the area of the filter 
outlet side. Noble metals such as platinum, palladium, etc. and mixtures 
may be used in their customary amounts as is known in the art. 
It should be noted that the suitability of the stated catalytically active 
substances for their stated use is related to their melting properties or 
their sublimation behavior. Thus, the overwhelming majority of the listed 
substances melt at temperatures below about 750.degree. C., and therefore 
at temperatures which lie in the range of the customary, achievable 
exhaust gas temperatures of Diesel engines, while the other compounds 
under these conditions exhibit a noticeable vapor pressure. 
For the practical use of the catalyst lowering the ignition temperature of 
Diesel soot, the active compound or its combination with a carrier 
material is applied on a filter element serving as structural reinforcer. 
The formation of washcoats and their use in catalyst technology is well 
known by now and therefore the conventional procedures may be used in this 
invention. 
The quantity of active substance to be applied in the individual case onto 
a carrier material or a filter element is less critical with regard to the 
lower limit than with regard to the upper limit. 
Generally as a minimum amount, it will be sufficient, whenever a carrier or 
a filtering structural reinforcer coated with carrier material contains 
the active substance in a quantity of 4 g/m.sup.2 of geometric surface. 
The upper range is limited by the maximum permissible drop in pressure 
over a given filter system. Accordingly, a generally suitable range is 
from a minimum of about 4 g/m.sup.2 to an upper amount which does not 
produce an unacceptably high pressure drop in the system. In case of a 
conventional monolithic filter body of cordierite with 11.85 cm diameter, 
15.25 cm length and 15.5 cells/cm.sup.2, 10-100, preferably 20-60 
g/m.sup.2 may be applied. 
As a structural reinforcer for the catalyst or a substance system 
containing it, conventional filter systems for the Diesel exhaust gases 
may be used, in which inlet and outlet channels for the exhaust gas are 
disposed such that in the case of the greatest possible filtering effort, 
a minimum of pressure loss occurs. 
As structural reinforcer, a packing of thermally stable metal- or mineral 
wool or a filter element according to German OS No. 29 44 841 or according 
to German OS No. 29 51 316 can be used. The use of a monolithic ceramic 
body, through which numerous flow channels pass, has turned out to be 
particularly favorable. The channel openings of which are provided in 
opposite end surfaces in such a way, that a channel open in one end 
surface is always closed in the opposite end surface. Such a filter 
therefore has the shape of a conventional, monolithic, structural 
reinforcer for catalysts and may consist of .alpha.-aluminum oxide or 
cordierite. The flow channels have macro-porous walls which are used as 
filter surfaces. In order to achieve this, a channel open at one end 
surface is always closed on the opposite end surface. The closure is 
accomplished with a ceramic plug, which is either sintered together with 
the material of the monolith or is glued together by means of a 
fire-resistant cement. The exhaust gas is therefore forced by 
obstructions, to flow through the macroporous walls of the channel as a 
result of which the Diesel soot is filtered and separated from the exhaust 
gas. 
Since the Diesel soot does not consist merely of carbon particles, but also 
contains about 20-30% adsorbed volatalizable hydrocarbons it may be of 
advantage to additionally impregnate the described catalytic Diesel 
filters with noble metal for the removal of said hydrocarbons. It has been 
proven to be particularly advantageous to coat the exhaust gas inlet side 
of the filter with the catalyst of the invention and the exhaust gas 
discharge side of the filter with a noble metal catalyst, consisting of at 
least one element of the platinum group of metals, possibly together with 
a catalysis-promoting metal oxide, such as .gamma.-Al.sub.2 O.sub.3. This 
has the additional advantage, that the carbon monoxide possibly developing 
during the burning off of the Diesel soot is converted by the noble metal 
catalyst into harmless carbon dioxide. The techniques employing noble 
metals and catalysis promoting oxides are well known in this art and may 
be used in combination with the present invention. 
The invention will be explained below on the basis of embodiments shown in 
more detail in the drawings wherein: 
FIG. 1a shows a top view and FIG. 1b is a section view of a Diesel filter 
in the form of a conventional monolithic structural reinforcer for 
catalysts made of cordierite with porous walls of the flow channels, 
wherein channels on the inlet end surface and the discharge end surface 
are selectively closed with a ceramic plug. These plugs are distributed 
such that a closed end of a channel is always opposite an open one; 
FIG. 2 shows a schematic presentation of a regeneration cycle of a filter, 
equipped according to the invention, with collecting phase, ignition phase 
and burning-off phase as well as with the pertinent temperature and 
differential pressure profiles. 
On the abscissa there is shown time sections in a manner of a model for one 
collecting-, ignition- and burning off phase. The course of temperature 
and back pressure (pressure differential over the filter) are assigned to 
the ordinate. The drop in pressure (solid line) rises in the collecting 
phase in case of a definite rpm and load to a predetermined value. Then 
the load and thus the temperature of the exhaust gas are regulated, until 
the temperature of the exhaust gas and the pressure drop between the 
exhaust gas entrance and the exit of the filter element does not rise 
significantly as is shown by the approximately horizontal portion of the 
curve for drop in pressure. In this nearly horizontal area essentially the 
amount of soot delivered from the diesel engine is no longer collected 
within the filter but is continuously ignited and burned off. In this case 
filtered out and burnt off soot are approximately in equilibrium. The 
temperature of the exhaust gas measured at the same time in front of the 
filter is called ignition temperature. For a complete regeneration of the 
filter the load is increased further, until a steep drop of the 
differential pressure over the filter has taken place by burning off the 
residual soot load to about the starting level. The load is then adjusted 
back to "normal operation".

The following examples illustrate the invention: 
EXAMPLE 1 
15.00 g of potassium perrhenate (KReO.sub.4) are dissolved in 1200 ml of 
de-ionized water in a beaker and the solution is divided in 4 parts. A 
commercially obtainable monolithic filter body of cordierite, as shown in 
FIG. 1, with 11.85 cm diameter, 15.25 cm length and 15.5 cells/cm.sup.2 is 
poured over from the direction of the front end surfaces with one part of 
the solution for impregnation and is subsequently dried at 200.degree. C. 
This process is repeated with the remaining 3 solution parts. 
EXAMPLE 2 
A solution of 40 g of potassium vanadate (KVO.sub.3) is prepared in 500 ml 
of de-ionized water. A filter body, as used in example 1, is impregnated 
with this solution by pouring it twice over the one end surface with 2 
hours of intermediate drying at 250.degree. C. A subsequent drying for 2 
hours likewise at 250.degree. C. results in the finished catalytic filter. 
EXAMPLE 3 
100 g of copper (I)-chloride (Merck No: 2738) are rolled with 140 ml of 
deionized water for 24 hours in a ball mill. Then this is diluted with 
de-ionized water up to 560 ml and half of this volume (280 ml) is poured 
over one end surface of a filter as used in example 1 and is subsequently 
dried in the drying oven for 4 hours at 100.degree. C. The copper chloride 
absorption amounts to 50 g. 
EXAMPLE 4 
A Diesel filter as used in Example 1 is coated from the direction of one 
front end surface with 45 g .gamma.-aluminum oxide from a 10% 
.gamma.-aluminum oxide dispersion, which contains the noble metals 
platinum and palladium in a weight ratio of 2:1 in the form of H.sub.2 
PtCl.sub.6 and PdCl.sub.2. With 45 g of aluminum oxide, a total of 2.58 g 
of platinum and 1.29 g of palladium are adsorbed by the filter. After 
subsequent drying at 250.degree. C. overnight and thermal decomposition of 
the noble metal compounds at 600.degree. C. over 2 hours in the muffle 
furnace, these were reduced for 2 hours at 550.degree. C. in a stream of 
hydrogen. After cooling down, the other side of the filter which is not 
yet coated is impregnated as in Example 2 and is designated as the input 
side of exhaust gas. 
EXAMPLE 5 
50 g of copper chromium oxide-.gamma.-aluminum oxide mixture (75 parts by 
weight:25 parts by weight), 4 g NiO (Merck No: 6723), 10 g of TiO.sub.2 (a 
pyrogenic titanium dioxide identified as P 25 of the Degussa company) and 
50 g V.sub.2 O.sub.5 (Merck No: 824), the latter doped with 3% K.sub.2 O 
(mixing with KOH, drying and tempering), are rolled with 4.5 ml of 
concentrated nitric acid and 400 ml of de-ionized water for 24 hours in a 
ball mill. Subsequently the ground material is diluted with de-ionized 
water to 1 liter total volume and a Diesel filter, used as in Example 1 is 
impregnated with 400 ml of this mixture from the direction of one side. 
After that it is dried for 3 hours at 300.degree. C. The absorption of 
solid substance amounts to a total of 45.6 g. The coated side is 
designated as the inlet side of the exhaust gas. 
EXAMPLE 6 
In a solution of 1.2 g Cu(NO.sub.3).sub.2.3H.sub.2 O (Merck No: 2751) in 
100 ml of de-ionized water, 50 g of TiO.sub.2 (a pyrogenic titanium 
dioxide identified as P 25 of Degussa company) are stirred in and the 
mixture is dried in the drying oven for 2 hours at 300.degree. C. 25 g are 
taken from the dried product and are rolled in a ball mill with 2.5 g 
V.sub.2 O.sub.5 (Merck No: 824) and 200 ml of de-ionized water, containing 
0.45 g KOH, for 24 hours. This is subsequently diluted with H.sub.2 O to 
280 ml and a Diesel filter as used in Example 1 is impregnated with it 
from the direction of one front side. After 2 hours of drying at 
300.degree. C. in the drying oven the coated side of the filter is 
designated as the inlet side for the exhaust gas. 
EXAMPLE 7 
A Diesel filter, as used in Example 1 is treated as described in Example 4, 
by coating it from one side with half of the washcoat, containing half the 
noble metal and by coating it from the other side (as in Example 2) with 
40 g V.sub.2 O.sub.5, doped with 5% by weight of K.sub.2 O. The side 
coated with the latter is designated as the input side of the exhaust gas. 
EXAMPLE 8 
A Diesel filter as the one used in Example 1 is coated as in Example 4 only 
with .gamma.-aluminum oxide and noble metals (platinum/palladium) and is 
subsequently subjected to the same activation as in Example 4. 
EXAMPLE 9 
100 g (NH.sub.4).sub.2 Ce(NO.sub.3).sub.6 (Rhone-Poulenc company) are 
dissolved in 4 liters of de-ionized water and 200 ml of concentrated 
nitric acid. To this 28 g NaVO.sub.3.H.sub.2 O (a product sold by the 
Riedel de Haen company and identified by No. 14203), dissolved in 500 ml 
of hot water, are added and the reaction mixture is heated to 
80.degree.-100.degree. C. The precipitated Ce(IV)-vanadate 
(2CeO.sub.2.V.sub.2 O.sub.5.nH.sub.2 O) is washed free of nitrate with 
warm water under repeated decanting is subsequently rolled with 400 ml of 
water overnight in a ball mill. The ground material is combined with water 
to 550 ml and a Diesel filter as used in Example 1 is impregnated with it 
from one side and excess suspension is blown out. After drying for 3 hours 
at 300.degree. C. the process is repeated with the same side of the 
filter. The adsorption of cerium (IV)-vanadate amounts to 30 g. 
EXAMPLE 10 
150 g of LiOH (Merck No. 5691) are dissolved in 3.5 liters of de-ionized 
water. A Diesel filter, as used in Example 1, is impregnated with the 
solution and is blown out with air for the removal of excess coating 
material. After 3 hours of drying at 200.degree. C. the adsorption of 
solid substance amounts to 21 g. 
EXAMPLE 11 
Comparative Example 
400 g of CuO (Merck No. 2761) are stirred into 1200 ml of de-ionized water 
and 31 ml of concentrated nitric acid and are rolled for 24 hours in a 
ball mill. 
A Diesel filter as used in Example 1 is impregnated with the suspension and 
is blown out with air for the removal of excess coating material. After 1 
hour of drying at 450.degree. C. the adsorption of solid substance amounts 
to 68 g. 
EXAMPLE 12 
Comparative Example 
500 g copper chromium oxide, 40 g NiO (Merck No: 6723) and 100 g TiO.sub.2 
(a pyrogenic titanium dioxide identified as P 25 of Degussa company) are 
stirred into 1500 ml of de-ionized water and 45 ml of concentrated nitric 
acid and are subsequently ground in a ball mill for 24 hours. A filter as 
used in Example 1 is impregnated on both sides with the suspension 
obtained and is blown out with air for the removal of excess coating 
material. After 1 hour of drying at 450.degree. C. the adsorption of solid 
substance amounts to 73 g. 
EXAMPLE 13 
40 g of vanadium pentoxide (Merck No: 824) are dispersed in a beaker in 600 
ml of de-ionized water and are stirred for 15 minutes with a Ultra-Turrax, 
a high intensity laboratory dispersion apparatus sold by the company Janke 
& Kunkel GmbH & Co. KG of Staufen, Germany brand high duty stirrer. A 
filter body as used in Example 1 is impregnated with the suspension thus 
obtained from the direction of one front side and excess suspension is 
blow out. This process is repeated once after a drying step of 1.5 hours 
at 300.degree. C. In addition, a 5 hour drying at 120.degree. C. takes 
place. The filter is now coated from the direction of one side with about 
37 g of V.sub.2 O.sub.5. 
Subsequently, the filter is impregnated from the direction of the same side 
with 250 ml of a 1% by weight KOH-solution. After 1 hour of drying at 
300.degree. C. in the drying furnace and a tempering of equal duration in 
the muffle furnace at 400.degree. C. the catalytic filter coated in this 
manner, is ready for use. 
EXAMPLE 14 
Application test 
Each of the filters produced in Examples 1-13, inclusive of an uncoated 
filter, was inserted into the exhaust gas stream of a Diesel engine and 
was checked as to their function for lowering the soot ignition 
temperature. The test parameters were: 
(a) Engine 
4 cylinder injection engine, Diesel 
1.6 l displacement 
40 kW power output 
(b) Soot loading phase of the filter 
rpm: 2900 min.sup.-1 
load: 33N 
oxygen content in the exhaust gas: ca. 14.2 Vol.% 
In the soot loading phase (b), the filter was exposed to the flow of 
exhaust gas, until a pressure difference of 150 mbar over the filter was 
reached. In order to determine the ignition temperature, the load was 
continuously increased at a constant rpm (2900 min.sup.-1), until the rise 
of the differential pressure over the filter came to a standstill (FIG. 
2). 
The temperature in the stream of exhaust gas reached thereby in front of 
the filter is defined as ignition temperature, since no further soot 
agglomeration takes place in the event of a constant differential 
pressure, i.e., an equilibrium ensues between filtered-out and burned-off 
soot. 
Table 1 shows these ignition temperature together with the pertinent load 
and the corresponding oxygen content in the exhaust gas for an attachment 
close to the engine of the filter (about 1 m behind the engine). Table 2 
contains the corresponding values for attachment located more distant from 
the engine (about 3-3.5 m behind the engine). 
The variable oxygen concentrations in the exhaust gas arise as a result of 
the load values needed for the burning-off or ignition temperatures. A 
high load is equivalent to a richer mixture and high exhaust gas 
temperature and thus also to a lower oxygen content in the exhaust gas. 
Further variations and modifications of the invention will be apparent to 
those skilled in the art and are intended to be encompassed by the claims 
appended hereto. 
TABLE 1 
______________________________________ 
Ignition 
Temp. load % O.sub.2 in the 
Example 
(.degree.C.) 
rpm (min.sup.-1) 
(N) exhaust gas 
______________________________________ 
* 505 2900 77 7.8 
1 476 " 71 8.6 
2 416 " 64 9.8 
3 300 " 55 --** 
5 412 " 62 10.0 
6 413 " 63 10.0 
7 406 " 63 10.3 
8 484 " 75 8.3 
9 425 " 65 10.0 
13 383 " 56 11.3 
______________________________________ 
*Filter body without catalyst 
**No exact determination of the O.sub.2portion in the exhaust gas 
possible 
TABLE 2 
______________________________________ 
Ignition 
Temp. load % O.sub.2 in the 
Example 
(.degree.C.) 
rpm (min.sup.-1) 
(N) exhaust gas 
______________________________________ 
* 520 2900 82 5.4 
1 455 " 74 7.7 
4 431 " 70 8.1 
8 498 " 80 6.2 
10 467 " 76 7.3 
11 513 " 82 5.3 
12 504 " 80 6.0 
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