Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Mixture is exhaust from internal-combustion engine
Reexamination Certificate
1997-05-15
2001-09-04
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Mixture is exhaust from internal-combustion engine
C423S213200, C431S007000
Reexamination Certificate
active
06284210
ABSTRACT:
SPECIFICATION
The present invention concerns a non-selective oxidation catalyst and the use thereof in the catalytic combustion of hydrocarbons, carbon monoxide, hydrogen or mixtures thereof.
Conventional combustion conducted in the presence of a flame, which is usually employed in processes for the combustion of hydrocarbons such as methane is a procedure which is difficult to control. It occurs in a well-defined range of air/hydrocarbon concentrations and, besides the formation of carbon dioxide and water, it results in the production of pollutants such as carbon monoxide and nitrogen oxides. Catalytic combustion produces few pollutants such as NO
X
and CO. In addition the introduction of a catalyst permits better control of total oxidation in a wide range of values in respect of the air/hydrocarbon ratio. They can be outside the limits of inflammability of conventional combustion. It may also be mentioned that it results in more compact apparatuses and that it makes it possible to burn a very wide variety of compounds.
As described in particular by D Reay in ‘Catalytic Combustion: Current Status and Implications for Energy Efficiency in the Process Industries. Heat Recovery Systems & CHP, 13, No 5, pages 383-390, 1993’ and D Jones and S Salfati in ‘Rev Gen Therm Fr No 330-331, pages 4101-406, June-July 1989’, there are multiple applications of catalytic combustion: radiant panels and tubes, catalytic heaters, gas turbines, co-generation, burners, catalytic sleeves for vapor-reforming tubes, production of hot gases in the field of heating by direct contact and reactors with catalytic plates. Because the standards relating to the NOx emitted by combustion processes are becoming more severe at an ever quickening rate, the catalytic combustion chamber can advantageously replace conventional burners which are the origin of high proportions of NOx. The operating conditions—highly oxidizing medium—of a catalytic combustion chamber are very remote from the applications of automobile post-combustion: treatment of the exhaust gases from petrol vehicles operating at the Level of richness 1 with a high content of NO
X
and treatment of the exhaust gases from diesel vehicles with a high proportion of particles and NOx. Those fundamental differences involve seeking dedicated formulations for combustion catalysts.
Combustion catalysts are generally prepared from a monolithic substrate of ceramic or metal, on which there is deposited a fine support layer formed by one or more refractory oxides of a surface area and porosity greater than those of the monolithic substrate. The active phase which is composed essentially of metals of the platinum group is dispersed on that oxide.
Thermal stability, low-temperature catalytic activity and stability of catalytic activity generally constitute the three main criteria for selection of the catalyst.
There are combustion catalysts which are more resistant to high temperature. In some combustion processes the catalysts may be subjected to very high temperatures which are often higher than 1000° C. In the course of their use at such high temperatures however it is found that the catalysts suffer from degradation which reduces their levels of catalytic performance. Sintering of the support and sintering of the active phase and/or encapsulation thereof by the support are part of the most generally quoted causes for explaining such degradation. In the case of such catalysts which operate at high temperature thermal resistance may become the predominant criterion, to the detriment of catalytic activity. The supports of those catalysts are generally alumina-based. It is known to the man skilled in the art that the drop in specific surface area can be effectively stabilized by a suitable doping agent. Rare earths and silica are often mentioned as being among the stabilizing agents with the best levels of performance in respect of the alumina. Catalysts prepared by that procedure are described inter alia in U.S. Pat. No. 4,220,559. In that document the catalyst comprises metals from the group of platinum or transition metals which are deposited on alumina, an oxide of a metal selected from the group formed by barium, lanthanum and strontium and an oxide of a metal selected from the group formed by tin, silicon, zirconium and molybdenum.
In addition, in order to limit sintering of the active metallic phase, it has been proposed that various stabilizing agents based essentially on oxides of transition metals may be added.
Thus, in US patent U.S. Pat. No. 4,857,499 the catalyst comprises a porous support in which the diameter of the pores is between 150 and 300 Å and of which the proportion by weight with respect to the substrate is preferably between 50 and 200 g/l, an active phase including at least 10% by weight, with respect to the porous support, of a precious metal selected from the group formed by palladium and platinum; a first promoter including at least one element selected from the group formed by lanthanum, cerium, praseodymium, neodymium, barium, strontium, calcium and oxides thereof, of which the proportion by weight, with respect to the porous support, is between 5 and 20%; a second promoter including at least one element selected from the group formed by magnesium, silicon and oxides thereof, of which the proportion by weight, with respect to the active phase, is less than or equal to 10%, and a third promoter including at least one element selected from the group formed by nickel, zirconium, cobalt, iron and manganese and oxides thereof, of which the proportion by weight, with respect to the active phase, is less than or equal to 10%. In addition said catalyst can be deposited on a monolithic substrate belonging to the group formed by cordierite, mullite, alpha alumina, zirconia and titanium oxide; the proportion by weight of porous support with respect to the volume of substrate being between 50 and 200 g/l.
In US patent U.S. Pat. No. 4,793,797 the catalyst comprises an inorganic support selected from the group formed by oxides, carbides and nitrides of elements belonging to groups IIa, IIIa and IV of the periodic system of elements or selected from the group formed by La-&bgr;-Al
2
O
3
, Nd-&bgr;-Al
2
O
3
, Ce-&bgr;-Al
2
O
3
or Pr-&bgr;-Al
2
O
3
, at least one precious metal selected from the group formed by palladium, platinum, rhodium and ruthenium, and at least one oxide of a base metal selected from the group formed by magnesium, manganese, cobalt, nickel, strontium, niobium, zinc, tin, chromium and zirconium, such that the atomic ratio of the base metal to the precious metal is between 0.1 and 10.
In regard to some thereof those catalysts exhibit increased durability with respect to the active metallic phase alone. However the doping agents used are adapted to very severe temperature conditions which may exceed 1000° C. They do not make it possible effectively to limit the deterioration in the levels of performance of the catalyst which occurs at moderate temperatures and which may be due to various causes that are different from those which are the origin of the deterioration at high temperatures.
Moreover, combustion catalysts have also been proposed, based on hexaaluminates containing manganese, affording a good compromise in terms of catalytic activity/thermal stability, as described particular in US patent U.S. Pat. No. 4,788,174. The oxidation catalyst which is thus proposed may be represented by the following formula:
A
1-z
C
z
B
x
Al
12-y
O
19-&agr;′
wherein:
A is at least one element selected from the group formed by Ba, Ca and Sr with (0.0≦0≦0.4);
B is at least one element selected from the group formed by Mn, Fe, Co, Ni, Cu and Cr with (x≦y≦2x);
C is K and/or Rb; and
&agr;=1½{X——z(X——Y)+xZ——3Y) in which X, Y and Z respectively represent the valencies of the elements A, C and B.
Such catalysts however are found to exhibit activity at low temperature which is inadequate to meet the requirements of a combustion process. In order to remedy that disadvantage it has been proposed that
Euzen Patrick
Mabilon Gil
Rebours Stephane
Tocque Eric
Griffin Steven P.
Ildebrando Christina
Institut Francais du Pe'trole
Millen White Zelano & Branigan
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