Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Silicon containing or process of making
Patent
1993-01-06
1995-04-25
Pal, Asok
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Silicon containing or process of making
502500, B01J 2104
Patent
active
054098785
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to non selective oxidation catalysts withstanding high temperatures, a method of preparation of these catalysts and their use for the catalytic combustion of the hydrocarbons, of the carbon monoxide, of the hydrogen and of their mixtures.
The combustion usually utilized in the processes of combustion of hydrocarbons such as methane produces nitrogen oxides NO.sub.x, carbon monoxide CO and unburnt residue. On the contrary the catalytic combustion also called "flameless" combustion produces little pollutants such as NO.sub.x and CO. This catalytic combustion moreover is of a more flexible utilization and may be carried out in explosive atmospheres.
The commonly used catalysts consist of a support made from an oxide of refractory metals of small specific surface upon which is deposited a fine alumina coating. The presence of a fine alumina coating allows to deposit the active phase consisting of noble metals in a state of high dispersion.
But it has turned out that in the course of the utilization of such catalysts at, high temperatures, the catalyst undergoes a degradation thus decreasing the catalytic performances. Among the causes generally contemplated to account for this deterioration of the catalyst, one may note the decrease of the number of active centers bound to an increase of the size of the metallic particles because of the high temperatures, of the presence of water and of oxygen.
Another cause justifying the degradation of the catalyst is a decrease of the specific surface area of the alumina also called sintering with its conversion into the alpha variety. This decrease of the specific surface area may both lead to an encapsulation of the metallic particles as well as to an exaltation of the sintering of the active phase.
Thus in order to limit this loss of the specific surface of the alumina, it has been proposed to add to the support comprising the alumina stabilizers in the course of the preparation.
Among these stabilizers, one may cite elements such as oxides of rare earths, essentially cerium and lanthanum, of zirconium, of baryum or of silicon.
The silicon is a particularly effective stabilizer. There has already been proposed to add to the alumina various silicon-based compounds in order to improve the stability of the support in alumina with respect to the thermal sintering. Thus it is known to add: 1,492,274 and in the French patent No. 2,249,852, application No. 184,506, document W. R. SYMES and E. RASTETTER, 24th International Colloquium on Refractories, Aachen.
Furthermore within the scope of the catalytic combustion it is known to introduce a noble metal from the platinum group on an alumina support stabilized by silicon.
Now the introduction of noble metals in a high dispersion state on a stabilized support assumes a strong interaction between the functional groupings of the support and the precursor of the active phase. This interaction depends essentially of the nature of these functional groupings. In the case of a stabilized alumina support, the hydroxyl groupings of the surface of the support are capable of reacting with precursors of the active phase in order to iramobilize this active phase on the surface of the support.
Therefore the interaction between the hydroxyl groupings present at the surface of the support and the precursor of the active surface is a function of the nature of these hydroxyl groupings and of their acido-basic character.
The present invention has therefore as its object to adapt the nature of the precursor of the active phase to the reactivity of the hydroxyl groupings present at the surface of the support in order to improve the interaction between these hydroxyl groupings and the precursor of the active phase.
This goal is reached with the catalyst according to the invention which comprises a support essentially based on alumina stabilized by silicon and a precursor of the active phase consisting of a noble metal M, the catalyst is characterized in that the said precursor of the active phase is selected among a cationi
REFERENCES:
patent: 4014948 (1977-03-01), Myers
patent: 4064154 (1977-12-01), Chandra et al.
patent: 4507401 (1985-03-01), Dubois et al.
patent: 4677095 (1987-06-01), Wan et al.
patent: 4766101 (1988-08-01), Nortier et al.
Blanchard Luc
Briot Patrick
Garbowski Edouard
Primet Michel
Gaz de France
Pal Asok
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