Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Sulfur or sulfur containing component
Reexamination Certificate
2000-02-02
2002-03-12
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Sulfur or sulfur containing component
C423S213200, C423S239100, C502S302000, C502S303000, C502S304000, C502S349000
Reexamination Certificate
active
06355220
ABSTRACT:
The present invention relates to a process for the treatment of exhaust gases from internal combustion engines operating with a fuel containing sulphur.
So-called multifunctional catalysts are currently used for the treatment of exhaust gases from internal combustion engines (automobile afterburning catalysis). Multifunctional catalyst is understood to mean catalysts capable of carrying out not only oxidation, in particular of carbon monoxide and hydrocarbons present in exhaust gases, but also reduction, in particular of nitrogen oxides also present in these gases (“three-way” catalysts). Zirconium oxide and cerium oxide today appear as two constituents which are particularly important and advantageous for this type of catalyst.
To be effective, these catalysts must first of all exhibit a high specific surface, even at high temperature. In addition, it is known that cerium makes it possible to ensure a buffering power with respect to variations in the oxygen content of the gas mixture to be treated and thus makes it possible to improve the performance of the catalyst with respect to the three main pollutants, namely CO, HC and NO
x
. This buffering power is evaluated by the capacity to store oxygen in an oxidizing environment and to restore it in a reducing environment. However, this oxygen storage capacity greatly decreases when the catalyst operates in a medium containing sulphur-containing compounds. Thus it is that some catalysts can prove to be inefficient when they are used in the treatment of exhaust gases from engines operating with fuels laden with sulphur-containing compounds.
The object of the invention is thus the development of a catalytic composition with an oxygen storage capacity which remains at an acceptable value in the case of the use of such fuels.
With this aim, the process according to the invention for the treatment of exhaust gases from internal combustion engines operating with a fuel containing sulphur is characterized in that use is made, as catalyst, of a composition based on a cerium oxide and on a zirconium oxide, in a cerium/zirconium atomic proportion of at least 1, and on at least one oxide chosen from yttrium, lanthanum and praseodymium oxide and which exhibits a specific surface, after calcination for 6 hours at 900° C., of at least 35 m
2
/g.
The invention also relates to a catalytic system for the treatment of exhaust gases from internal combustion engines operating with a fuel containing sulphur which is characterized in that it comprises a composition of the abovementioned type.
Other characteristics, details and advantages of the invention will become still more fully apparent on reading the following description, as well as the various concrete but non-limiting examples intended to illustrate it.
The compositions which can be used in the context of the process of the invention will now be described.
These compositions are based on a cerium oxide and on a zirconium oxide and on at least one oxide chosen from yttrium, lanthanum and praseodymium oxide. The cerium/zirconium atomic proportion is at least 1.
The compositions of the invention can correspond to the formula Ce
x
Zr
y
M
z
O
2
in which M represents at least one element chosen from the group comprising yttrium, lanthanum and praseodymium. In this formula, z is greater than 0 and preferably exhibits a value of at most 0.3 and which can more particularly be between 0.02 and 0.2. The x/y ratio can be between 1 and 19, more particularly between 1 and 9 and more particularly still between 1.5 and 4, the values of the limits, other than 0, being included and x, y and z being linked by the relationship
x+y+z=
1.
Another characteristic of the compositions of the invention is their specific surface. Thus, they exhibit a specific surface, after calcination for 6 hours at 900° C., of at least 35 m
2
/g. This surface can more particularly be at least 40 m
2
/g. It can more particularly still be at least 45 m
2
/g.
The compositions of the invention can also exhibit surfaces which still remain significant even after calcination for 6 hours at 1000° C. These surfaces can be at least 14 m
2
/g, more particularly at least 20
2
/g and more particularly still at least 30 m
2
/g.
The compositions of the invention can advantageously exist in the form of a solid solution. The X-ray diffraction spectra of these compositions in fact reveal, within the latter, the existence of a single homogeneous phase. For the compositions which are the richest in cerium, this phase corresponds in fact to that of a crystalline ceric oxide CeO
2
, the unit cell parameters of which are more or less offset with respect to a pure ceric oxide, thus reflecting the incorporation of zirconium and, if appropriate, of the other element in the crystal lattice of the cerium oxide and thus the preparation of a true solid solution.
The process for the preparation of the compositions described above will now be described.
The first stage of the processing according to the invention consists in preparing a mixture in liquid medium, generally in the aqueous phase, containing at least one cerium compound, at least one zirconium compound and at least one yttrium, lanthanum or praseodymium compound. This mixture is prepared by using a zirconium solution.
This zirconium solution can originate from the attach of acid on a reactant comprising zirconium. Mention may be made, as an appropriate reactant, of zirconium carbonate, hydroxide or oxide. The attack can be carried out with an inorganic acid, such as nitric acid, hydrochloric acid or sulphuric acid. Nitric acid is the preferred acid and the use of a zirconyl nitrate originating from the attack of nitric acid on a zirconium carbonate may thus be very particularly mentioned. The acid can also be an organic acid, such as acetic acid or citric acid.
According to the invention, this zirconium solution must exhibit the following characteristic. The amount of base necessary to reach the equivalent point during an acid/base titration of this solution must confirm the condition that, as a molar ratio, OH
−
/Zr≦1.65. More particularly, this ratio can be at most 1.5 and more particularly still at most 1.3. Generally, the specific surface of the composition obtained has a tendency to increase when this ratio decreases.
The acid/base titration is carried out in a known way. In order for it to be carried out under optimum conditions, a solution which has been brought to a concentration of approximately 3×10
−2
mol per litre, expressed as elemental zirconium, can be titrated. A1N sodium hydroxide solution is added thereto with stirring. Under these conditions, the equivalent point (change in the pH of the solution) is determined in a clear-cut way. This equivalent point is expressed by the OH
−
/Zr molar ratio.
Mention may particularly be made, as cerium compounds, of cerium salts such as cerium(IV) salts, such as nitrates or ceric ammonium nitrates for example, which are particularly well suited in this instance. Ceric nitrate is preferably used. The solution of cerium(IV) salts can contain cerium in the cerous state but is preferable for it to contain at least 85% of cerium(IV). An aqueous ceric nitrate solution can, for example, be obtained by reaction of nitric acid with a ceric oxide hydrate prepared conventionally by reaction of a solution of a cerous salt, for example cerous nitrate, and of an aqueous ammonia solution in the presence of hydrogen peroxide. Use can also be made of a ceric nitrate solution obtained according to the process of electrolytic oxidation of a cerous nitrate solution as described in the document FR-A-2,570,087, which can constitute an advantageous starting material.
It will be noted here that the aqueous solution of cerium(IV) salts can exhibit a degree of initial free acidity, for example a normality varying between 0.1 and 4N. According to the present invention, it is just as possible to use an initial solution of cerium(IV) salts effectively exhibiting a degree of free acidity as mentioned above as a solution which would
Blanchard Gilbert
Visciglio Valerie
Hendrickson Stuart L.
McViegh Kevin E.
Rhodial Chimie
Strickland Jonas N.
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