Process for reducing NO.sub.x from exhaust fumes

NO.sub.x, where x is 1 and 2, in exhaust gases is reduced over heterogeneous catalysts with hydrocarbons, carbon monoxide, hydrogen or mixtures thereof in the presence of oxygen from 100 to 650.degree. C. and an absolute pressure of from 0.5 to 50 bar by a process in which the heterogeneous catalysts used are bimodal or polymodal compounds of the general formula I EQU A.sub.1-x M.sub.2 O.sub.4 (I), which, if required, are doped with rare earth metals, noble metal, titanium, vanadium, molybdenum, tungsten or mixtures thereof, and where PA1 A is magnesium, calcium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, tin or mixtures thereof, PA1 M is aluminum, gallium, indium, tin, chromium, manganese, iron, cobalt, nickel, copper, zinc or mixtures thereof and PA1 x is from 0 to 0.99, whose oxygen atoms are in a cubic close packed arrangement and A is present in the tetrahedral sites and M in the octahedral sites, it also being possible for A to occupy octahedral sites if an equivalent amount of M occupies tetrahedral sites.

The present invention relates to a process for the catalytic reduction of 
NO, NO.sub.2 or mixtures thereof in exhaust gases, in particular 
combustion exhaust gases and especially combustion exhaust gases from 
internal combustion engines which are operated with air, preferably in 
excess, for example diesel and lean-mix engines, with the use of 
hydrocarbons, carbon monoxide, hydrogen or mixtures thereof as reducing 
agents. 
The non-prior published DE-A-44 20 932 discloses that a specific 
copper/zinc spinel may be used for reducing NO.sub.x from automobile 
exhaust gases. These copper/zinc spinels are very difficult to prepare in 
a reproducible manner and exhibited insufficient resistance to SO.sub.2 
which is contained in the exhaust gases of diesel engines. 
EP-A-42 471 discloses a process for the catalytic oxidation of carbon 
monoxide to carbon dioxide, in which the catalyst contains the metals 
copper, zinc and aluminum as metal oxides. At least 60% of the copper 
present is bound to the aluminum oxide as copper oxide/aluminum oxide 
spinel. From 1 to 20% by weight of zinc oxide are arranged in the cavities 
of the porous spinel. This catalyst is used in ammonia production for 
low-temperature conversion of CO. 
DE-A-43 01 470 discloses a CuAl.sub.2 O.sub.4 spinel which is combined with 
tin, lead and an element of Main Group II of the Periodic Table of the 
Elements as an oxide or a salt or in elemental form and then calcined to 
give a spinel. The known spinel of this type is used for decomposing 
N.sub.2 O. 
The emissions of oxides of nitrogen in the modern industrial states are 
governed by the emitters comprising traffic and power stations and 
industrial plants fired by means of fossil fuels. While the power station 
and industrial emissions are declining through the construction of 
appropriate waste gas purification plants, the contribution of traffic is 
becoming more and more prevalent. 
In the case of gasoline engines, the oxides of nitrogen can be reduced in a 
known manner by means of a three-way catalyst in a stoichiometric 
procedure, the uncombusted or partially oxidized components of the exhaust 
gas being ideally available in a stoichiometric ratio as a reducing agent 
for the resulting oxides of nitrogen. 
In the diesel engine and in the lean-mix engine operated 
superstoichiometrically with an excess of air, this method of reducing 
oxides of nitrogen is in principle not possible owing to the 
superstoichiometric procedure. On the other hand, emissions of oxides of 
nitrogen from diesel engines, in particular in the direct injectors which 
have advantageous consumption and predominate in the commercial vehicle 
sector, account for a high proportion of the total emissions. 
The advantageous consumption of lean-mix engines compared with engines with 
lambda regulation and three-way catalyst favors the increased use of 
lean-mix engines. 
Noble metal-containing catalysts which, however, produce nitrous oxide 
(N.sub.2 O) in the reduction of the oxides of nitrogen in the presence of 
oxygen are also known. 
It is an object of the present invention to remedy the abovementioned 
disadvantages. 
We have found that this object is achieved by a novel and improved process 
for the reduction of NO.sub.x, where x is 1 and 2, in exhaust gases over 
heterogeneous catalysts with hydrocarbons, carbon monoxide, hydrogen or 
mixtures thereof in the presence of oxygen at from 100 to 650.degree. C. 
and an absolute pressure of from 0.5 to 50 bar, wherein the heterogeneous 
catalysts used are bimodal or polymodal compounds of the general formula I 
EQU A.sub.1-x M.sub.2 O.sub.4 (I), 
which, if required, are doped with rare earth metals, noble metals, 
titanium, vanadium, molybdenum, tungsten or mixtures thereof, and where 
A is magnesium, calcium, titanium, vanadium, chromium, manganese, iron, 
cobalt, nickel, copper, zinc, tin or mixtures thereof, 
M is aluminum, gallium, indium, tin, chromium, manganese, iron, cobalt, 
nickel, copper, zinc or mixtures thereof and 
x is from 0 to 0.99, 
whose oxygen atoms are in a cubic close packed arrangement and A is present 
in the tetrahedral sites and M in the octahedral sites, it also being 
possible for A to occupy octahedral sites if an equivalent amount of M 
occupies tetrahedral sites, 
preferably the abovementioned heterogeneous catalysts, with the proviso 
that they are not spinels which fulfill the following condition: 
EQU Cu.sub.A Zn.sub.B Al.sub.C O.sub.4, 
where A+B+C=3 and A&gt;0, B&gt;0 and C&gt;0.

The novel process can be carried out as follows: 
The exhaust gas of a gas turbine, of a diesel engine or of a lean-mix 
engine is passed, after emerging, for example, from the gas turbine, from 
the engine or from the turbocharger, at from 100 to 650.degree. C., 
preferably from 150 to 550.degree. C., particularly preferably from 200 to 
500.degree. C., in particular from 300 to 450.degree. C. and at an 
absolute pressure of from 0.5 to 50, preferably from 1 to 20, particularly 
preferably from 1 to 6, in particular from 1 to 3, bar and in general at a 
GHSV (Gas Hourly Space Velocity) of from 1000 to 200,000, preferably from 
2000 to 150,000, particularly preferably from 5000 to 100,000, (l (S.T.P.) 
of gas)/(l of catalyst.multidot.h), preferably at the pressure prevailing 
at the gas turbine, at the engine outlet or at the turbocharger outlet, 
together with hydrocarbons, carbon monoxide, hydrogen, alcohols, such as 
methanol or ethanol, or mixtures thereof, over a novel catalyst. The 
catalyst may be in the form of pellets but is preferably present in a 
honeycomb structure. The honeycomb structure of the catalyst can be 
produced by extruding the catalytic material to give honeycombs having 
triangular, quadrilateral, polygonal or circular honeycomb channels or by 
coating carriers shaped in this manner with the catalytic material. 
Further embodiments are the coating and/or the impregnation of undulating 
metal sheets or fiber mats, which, for example, can be rolled or laid 
together to give a honeycomb structure, with the catalytically active 
material. The geometry of the honeycomb structure should be chosen so that 
the soot contained in the exhaust gas is not deposited on the honeycomb 
structure, thus leading to blockage of the honeycomb. During passage of 
the gas through the catalyst, oxides of nitrogen are reduced and at the 
same time the hydrocarbons and CO contained in the gas are oxidized. For 
the reduction of the oxides of nitrogen, hydrocarbons, preferably motor 
fuel, are metered into the exhaust gas; advantageously, however, the 
hydrocarbons contained in the exhaust gas and the CO present may also be 
sufficient for the reduction. 
Suitable catalysts are those which can be prepared as follows: 
The preparation of bimodal or polymodal, preferably bimodal, trimodal and 
tetramodal, particularly preferably bimodal and trimodal and in particular 
bimodal, heterogeneous catalysts is carried out as a rule according to the 
following principle: 
Oxide particles of the general composition M.sub.2 O, MO, M.sub.2 O.sub.3 
or mixtures thereof, which have a particle size of from 1 to 1000 .mu.m, 
preferably from 5 to 600 .mu.m, particularly preferably from 10 to 500 
.mu.m, in particular from 20 to 200 .mu.m, may be combined with 
hydroxide-containing particles, for example of the general composition 
M(OH), M(OH).sub.2, M(OH).sub.3 and MO(OH), for example by mechanical 
mixing, grinding in an edge mill or spray drying. These mixtures can be 
brought into a plastic foam, for example by kneading or grinding in an 
edge mill, can be extruded to give a molding (for example a solid 
extrudate, a hollow extrudate, a star extrudate or a honeycomb) and as a 
rule can be calcined. After a calcination, the resulting oxide, which as a 
rule has the general composition M.sub.2 O, MO, M.sub.2 O.sub.3 or a 
mixture thereof, possesses a bimodal or polymodal pore distribution. 
The component A can be either applied by impregnation or added to the 
mixture described above and the mixture kneaded and extruded. Calcination 
gives an oxide of the formal composition AM.sub.2 O.sub.4, where A may 
assume the valencies +2, +4 or +6 and M the valencies +1, +2 or +3. These 
oxides, which essentially have a spinel, inverse spinel or defect spinel 
structure, have bimodal or polymodal pore distributions. 
Preferably, one or more further elements of A, i.e. A', A", A'" etc., may 
be added to the oxides of the formal composition AM.sub.2 O.sub.4, for 
example by impregnation, mechanical mixing or spraying, one or more 
further calcination steps, preferably one further calcination step, giving 
a solid (oxide) which corresponds to the general composition (AA')M.sub.2 
O.sub.4, M(AA')M)O.sub.4, (AA').sub.0.99-0.01 M.sub.2 O.sub.4, 
(AA'A")M.sub.2 O.sub.4, M(AA'A")M)O.sub.4, (AA'A").sub.0.99-0.01 M.sub.2 
O.sub.4, (AA'A"A'")M.sub.2 O.sub.4, M(AA'A"A'")M)O.sub.4, 
(AA'A"A'").sub.0.99-0.01 M.sub.2 O.sub.4. These solids, which are the 
novel ready-prepared and used catalysts, have a bimodal or polymodal pore 
distribution. 
The calcinations are carried out as a rule at from 300 to 1300.degree. C., 
preferably from 500 to 1200.degree. C., particularly preferably from 600 
to 1100.degree. C., and at from 0.1 to 200, preferably from 0.5 to 10, 
bar, particularly preferably at atmospheric pressure. 
The oxidic solids obtained are partially or completely, i.e. to an extent 
of from 1 to 100, preferably from 10 to 90, particularly preferably from 
20 to 70, % by weight, spinels AM.sub.2 O.sub.4, inverse spinels 
M(AM)O.sub.4 or optionally defect spinels of the composition A.sub.l-x 
M.sub.2 O.sub.4 in an M.sub.2 O.sub.3 matrix or optionally defect inverse 
spinels of the composition M(A.sub.l-x M)O.sub.4 in an M.sub.2 O.sub.3 
matrix. These solids are distinguished by the fact that the oxygen atoms 
have a cubic close packed arrangement and A are present in the tetrahedral 
sites and M in the octahedral sites, it also being possible for A to 
occupy octahedral sites if the equivalent amount of M occupies tetrahedral 
sites. 
Suitable elements M in the oxides of the composition A.sub.l-x 
O.multidot.M.sub.2 O.sub.3 are aluminum, gallium, indium, tin, titanium, 
chromium, manganese, iron, cobalt, nickel, copper, zinc or mixtures 
thereof, preferably aluminum, gallium, manganese, iron, cobalt, nickel or 
mixtures thereof, particularly preferably aluminum and gallium, in 
particular aluminum. 
x is from 0 to 0.99, preferably from 0 or 0.6 to 0.01, particularly 
preferably from 0 or 0.5 to 0.05. 
Suitable elements A in the oxides of the composition A.sub.l-x 
O.multidot.M.sub.2 O.sub.3 are magnesium, calcium, scandium, titanium, 
vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, tin or 
mixtures thereof, preferably in the oxidation state +2, such as Mg.sup.2+, 
Ca.sup.2+, Ti.sup.2+, V.sup.2+, Cr.sup.2+, Mn.sup.2+, Fe.sup.2+, 
Co.sup.2+, Ni.sup.2+, Zn.sup.2+, Sn.sup.2+ and Cu.sup.2+ or mixtures 
thereof, particularly preferably Mg.sup.2+, Ca.sup.2+, Mn.sup.2+, 
Fe.sup.2+, Co.sup.2+, Ni.sup.2+, Zn.sup.2+, Sn.sup.2+ and Cu.sup.2+ or 
mixtures thereof, in particular Zn.sup.2+, Mg.sup.2+, Co.sup.2+, Ni.sup.2+ 
and Cu.sup.2+ or mixtures thereof. These may be used in the form of an 
element (metal), or as oxidic or salt-like compounds. Examples of 
salt-like compounds are carbonates, hydroxides, carboxylates, halides, 
halogenates, nitrites, nitrates, sulfites, sulfates, phosphite, 
phosphates, pyrophosphates, halites, and basic carbonates, preferably 
carbonates, hydroxides, carboxylates, nitrates, nitrites, sulfates, 
phosphates and basic carbonates, particularly preferably carbonates, 
hydroxides, basic carbonates and nitrates. 
The novel catalysts are bimodal. For the purposes of the present invention, 
bimodal means that they contain mesopores (pore diameter&lt;50 nm) and 
macropores (pore diameter 0.05-50 .mu.m). The proportion of mesopores is 
from 40 to 99, preferably from 50 to 98, particularly preferably from 55 
to 95, % by volume, based on the total pore volume of the novel catalysts. 
The proportion of macropores is from 1 to 60, preferably from 2 to 50, 
particularly preferably from 5 to 45, % by volume, based on the total pore 
volume of the novel catalysts. The novel catalysts may also be polymodal, 
i.e. they additionally contain pores in the range&gt;50 .mu.m. In this case, 
the proportion of pores having a diameter&gt;50 .mu.m is in general from 0.1 
to 20, preferably from 1 to 15, particularly preferably from 2 to 10, % by 
volume, the abovementioned percentages for mesopores and macropores also 
being applicable for this case but being based on the total pore volume 
minus the volume of the pores having a diameter of &gt;50 .mu.m. 
The BET surface areas of the novel catalysts are from 1 to 350, preferably 
from 10 to 200, particularly preferably from 30 to 140, m.sup.2 /g and the 
porosities are from 0.01 to 0.8, preferably from 0.05 to 0.7, particularly 
from 0.1 to 0.6, ml/g. 
EXAMPLES 
Preparation of the Catalyst 
Example A 
A mixture of 173 g of Al.sub.2 O.sub.3 (Puralox SCF.RTM. from Condea), 96 g 
of AlOOH (Pural SB.RTM. from Condea), 91 g of Cu(NO.sub.3).sub.2 *3H.sub.2 
O (from Merck) and 116 g Co(NO.sub.3).sub.2 *6H.sub.2 O was kneaded with 
14 ml of formic acid (dissolved in 130 ml of H.sub.2 O) for 1 h, extruded 
to give 3 mm solid extrudates, dried, and calcined for 4 hours at 
800.degree. C. at atmospheric pressure. 
The material obtained after the calcination has a surface area (measured 
according to BET) of 106 m.sup.2 /g. It formally has the composition 
Cu.sub.0.15 Co.sub.0.17 Al.sub.2 O.sub.4 and shows the typical diffraction 
lines of a spinel in the X-ray diffraction pattern. 
146 g of the solid described above and having the composition Cu.sub.0.16 
Co.sub.0.17 Al.sub.2 O.sub.4 (water absorption: 0.5 ml/g) were impregnated 
twice with, in each case, 36.5 ml of an aqueous nitrous acid solution (pH 
3) which contained 34 g of Zn(NO.sub.3).sub.2.6H.sub.2 O and then left at 
room temperature for one hour. The impregnated carrier was dried at 
120.degree. C. to constant weight and finally calcined for 4 hours at 
600.degree. C. 
The result was a catalyst of the formal composition Zn.sub.0.39 Cu.sub.0.16 
Co.sub.0.17 Al.sub.2 O.sub.4, which had the X-ray diffraction pattern of a 
spinel. The surface area of the catalyst (according to BET) was 66 m.sup.2 
/g. The pore radius distribution was measured by the mercury porosimetry 
method (DIN 66 133). About 60% by volume of the pore volume are accounted 
for by pores having a diameter of &lt;0.05 .mu.m and about 30% by volume by 
pores in the range of 0.6-5 .mu.m (FIG. 1). In FIG. 1, the cumulative pore 
volume (cumulative intrusion) in ml/g is plotted against the pore diameter 
in .mu.m. 
Example 1 
The spinel used was a cobalt/copper/zinc/aluminum spinel of the composition 
Zn.sub.0.39 Cu.sub.0.16 Co.sub.0.17 Al.sub.2 O.sub.4. 10 g of the spinel 
in the form of chips of the fraction from 1.6 to 2.0 mm were initially 
taken in a vertical quartz reactor (diameter 20 mm, height about 500 mm), 
in which a gas-permeable frit was arranged for holding the sample in the 
center of said reactor. The bed height was about 15 mm. Arranged around 
the quartz reactor was an oven which heated the central part of the 
reactor over a length of about 100 mm, temperatures up to 550.degree. C. 
being achievable. 
A gas mixture was passed through the catalyst and a GHSV of about 10,000 (l 
(S.T.P.) of gas)/l of catalyst.multidot.h), which mixture consisted of 
1000 ppm of NO, 1000 ppm of propene, 10% of oxygen and argon (remainder) 
as carrier gas. The NO concentration was measured by means of a gas 
detector downstream of the reactor, any NO.sub.2 formed being reduced to 
NO in a converter before the detection. At the same time, oxidation of 
hydrocarbons to CO.sub.2 was observed by measuring the CO.sub.2 content by 
the gas detector. The result of the measurement is shown in a graph in 
FIG. 2. The curve of the NO.sub.x and of the CO.sub.2 content in ppm is 
plotted as a function of the temperature, the NO.sub.x concentration being 
indicated by the thicker line. The graph shows a substantial decrease in 
the NO.sub.x concentration with increasing temperature, said concentration 
passing through a broad minimum in the range of 300-450.degree. C. and 
then increasing again. At the same time, the hydrocarbons are converted 
into CO.sub.2, as indicated by the increase in the CO.sub.2 concentration. 
Advantageously, these are roughly the temperatures which can occur in an 
exhaust gas line of an internal combustion engine. Investigations into 
this catalyst furthermore indicated high resistance to NO.sub.x, water and 
carbon dioxide.