FR2629366A2 - Application of silicon carbide of high specific surface to catalytic reactions at elevated temperature - Google Patents

Abstract

Application of silicon carbide of high specific surface of at least 100 m<2> per gram, according to claim 1 of the main patent application, to the use of catalytic reactions at elevated temperature, higher than 500 DEG C and capable of being between 650 and 800 DEG C, such as the supports for catalysts intended for exhaust systems of internal combustion engines, and for controlled oxidation of hydrocarbons of low molecular mass, especially methane, to hydrocarbons of higher molecular mass and longer carbon chain. Active carbon of high specific surface, reacted with silicon monoxide, is preferably doped by impregnation with a solution of a salt of a polyvalent metal such as U, Ce, Ti, Zr, Hf and lanthanides.

Description

APPLICATION W SPECIFIC LARGE SURFACE SILICON CARBIDE
TO CATALYTIC RECTIONS AT HIGH TEMPERATURE
TECHNICAL FIELD OF THE INVENTION
The present invention constitutes a second addition to main patent application 87-14742, filed on October 19, 1987, and more precisely a new application of silicon carbide with a large specific surface area, at least equal to 100 m 2 / g, described and claimed in the main demand, as a catalyst support for reactions at high temperature, greater than 5000C and preferably between 650 and 8000C.

REMINDER OF THE SUBJECT OF THE MAIN PATENT APPLICATION
In the main patent application, a process for preparing silicon carbide with a large specific surface area, at least equal to, has been described. 100 square meters per gram, process consisting in generating silicon monoxide vapors in a first zone of a reactor and then in contacting these vapors, in a second zone of the reactor with active carbon in the divided state, at the surface specific at least equal to 200 m2 per gram. The silicon carbide thus produced has a specific surface area at least equal to 100 m 2 per gram, and being able to reach up to 400 m 2 per gram, in particular when it is doped with uranium.

This silicon carbide with a large specific surface is particularly suitable as a support for catalysts in petrochemicals, replacing traditional supports such as alumina, because they are more resistant to "poisoning" by impurities and especially to regeneration cycles. high temperature, and finally they allow easier recovery of the precious metals constituting the catalyst
SUBJECT OF THE INVENTION
The Applicant has found that the silicon carbide with a large specific surface area, which is the subject of the main invention, constitutes an excellent catalyst support for numerous chemical reactions at high temperature, taking place in the presence of a catalyst, other than those that this involves petrochemicals, and which did not appear to be obvious to those skilled in the art.Among these new applications, we can cite without limitation - the exhaust pipes of internal combustion engines, destined for
convert carbon monoxide and any unburned hydrocarbons
into carbon dioxide and water and the lower nitrogen oxides into dioxide,
and for which we currently use activated alumina with
rhodium and / or platinum salts.

Silicon carbide with a high specific surface (> 100 m2 / g) can
be activated by impregnation with a rhodium salt (such as chloride)
and / or platinum (salt of hexachloroplatinic acid) at a concentration
equal and even lower (with equal efficiency) to those that we use
implemented in the case of alumina.

In addition to a reduction in the cost price, there is a lifespan
significantly increased usefulness of catalytic converters using carbide
silicon according to the invention as catalyst support. In particular,
such pots are insensitive to sudden rises in temperature
exhaust gases.

- the so-called "controlled oxidation" catalysts of low-carbon hydrocarbons.
molecular mass such as methane, which are often oxide based
lithium and / or magnesium, which make it possible to obtain the conversion
in hydrocibures with longer carbon chain and molecular mass
higher, at a temperature of the order of 650 to 8000C. In this
application, catalysts supported by alumina rapidly lose
their effectiveness as a result of a rapid decrease in the specific surface
alumina. On the contrary, silicon carbide resists remarkable
is lying; tests have shown that the specific surface of a sample,
initially equal to 120 m2 / g was still equal to 60 m2 / g after heating
extended to 10000C. In the case of a silicon carbide doped with uranium,
the initial surface of 200 m2 / g is still 120 to 130 m2 / g after heating
fage extended to 10000C. An active alumina thus treated sees its surface
specific reduced to a few m2 / g.

In these two high temperature applications, silicon carbide with a large specific surface area shows remarkable resistance to poisoning and aging, and very good recovery of its specific surface after regeneration at a temperature which can reach 10000C.

INFLUENCE OF ACTIVE CARBON DOPING ON THE SPECIFIC CARBIDE SURFACE
SILICON
To show the influence of the doping of active carbon with a large specific surface area used as a base material to produce silicon carbide with a large specific surface area (by reaction with silicon monoxide), the following tests were carried out A- Doping at uranium.

The active carbon, with a large specific surface area, was doped with uranium by impregnation with an alcoholic solution of uranyl acetylacetate, so as to obtain a concentration by weight of uranium (in the initial active carbon) of 8 to 15%. The carbon thus treated was calcined under argon at 5000C then introduced into the reactor, and reacted with silicon monoxide, according to the invention. The specific surface of the silicon carbide thus obtained was then measured, before calcination and after calcination in air for 2 hours at 10,000 C. The results are as follows:% U by weight Q 8.5 9.8 15 m2 / g before calcination 197 425 410 279 mZ / g after 2 hours at 10000C 59 109 131 76
It appears that the optimum uranium concentration in the activated carbon is around 8 to 10% by weight, which corresponds to an actual content in the silicon carbide of the order of 13 to 14% by weight.

X-ray examination shows that uranium is found in silicon carbide in the form of U307 before calcination and in the form of U308 after calcination for 2 hours at 10000C.

B - Cerium doping
In the same way, we carried out the doping with cerium of the active carbon, by impregnation with water-soluble cerium salts such as nitrate.
Ce (NO3) 2, 6 H2O or ammoniacal nitrate <NH4> 2 Ce (NO3) 6.

The results are as follows% Ce in active C 3 5 5 5 (by weight) nitrate ammoniacal nitrate m2 / g before calcination 303 376 292 320 m2 / g after 2 hours at 10000C 73 94 134 141
It is observed that the nature of the cerium salt has a certain influence on the final result, at an equal concentration of cerium, probably by differences in the penetration of the solution into the pores of the active carbon.

The two tests with ammoniacal nitrate correspond to slight modifications of the parameters of the SiO on C reaction producing SiC.

It is also observed that doping with cerium is at least as effective as doping with uranium. Other dopants can be used, and in particular salts of titanium, zirconium, hafnium, and lanthanides.

APPLICATION EXAMPLE
The invention has been implemented under conditions simulating the operation of catalytic exhaust pipes for internal combustion engines under conditions conforming to the European standards under development.

4 devices were prepared simulating a catalytic converter, the catalyst being a platinum-rhodium mixture, the active phase concentration being 0.2% of Pt and 0.02% by weight of Rh.

The 4 catalysts were respectively deposited by conventional methods on - active alumina at 235 m2 / g - uncalcined SiC at 125 m2 / g - SiC calcined at 800 C, at 30 m2 / g - calcined SiC at 10000C at 18 m2 / g.

A gas simulating an engine exhaust gas was sent to these catalysts, with a flow rate, per minute, of 58.5 cm3 of CO + N2, 350 ppm of
C3Hg, 450 ppm NO.

The efficiency of these 4 catalysts was measured from the point of view of the conversion of CO to CO2, and of the nitrogen sub-oxides to NO2 and of the oxidation of the unburnt particles, as a function of the temperature.

The results are given in the table below. We see that the
SiC calcined at 8000C has an efficiency quite comparable to that of alumina. It should be added that SiC also has a very great superiority over alumina, which is the ease of recovery of the catalyst, which on the contrary poses very difficult problems in the case of alumina.

Temperature T at x% conversion GAS 1OC) CO NOx 'C3H8
T-10% Ref. A1203 2110C 2310c 2510
SiC before calcination 252 249 304
SiC after calcination 231 215 275
SiC after calcination 10,000 304 305 348
(calcined in air, 15H)
T-25% Ref. A1203 230 244 264
SiC before calcination 271 269 307
SiC after calination 8000 244 232 280
SiC after calcination 10,000 322 322 362
(calcined in air, 3 p.m.)
T-50% Ref. A1203 245 252 350
SiC before calcination 284 282 347
SiC after calcination 8000 259 251 385
SiC after calcination 10,000 338 336 380
(calcined in air, 3 p.m.)
T-75% Ref. A1203 257 265 394
SiC before calcination 296 292 412
SiC after calcination 8000 266 265 462
SiC after calcination 10,000 352 349 404
(calcined in air, 3 p.m.)
T-90% Ref.A1203 270 279 419
SiC before calcination 304 301 450
SiC after calcination 8000 '270 272 488
SiC after calcination 10,000 364 360 429
(calcined in air, 3 p.m.)
CONVERSION RATE t
Efficiency in% Surf.Spec. CO NOx C3H8
Reference (@@ 203) 235 m2 / g 71.0 71.0 58.7
SiC before calcination 125 62.1 63.9 54.4
SiC after cullet. 8000 30 69.7 73.1 52.0
It is noted that the efficiency of the catalyst deposited on SiC calcined at 800 C, at 30 ira2 / 9, is substantially equal to that of the catalyst on alumina with regard to CO and NOx, and barely lower with regard to unburnt particles.
These results can be further improved by optimizing the doping (with uranium, cerium or other) as a function of the goal to be achieved, since it is easy, by implementing the invention, to use as a support of the active phase of the catalyst, SiC retaining, at 8000C, a specific surface area greater than 300 m2 / g, and, at 10000C, still greater than 100 m2 / g.

Claims (2)

1. Application of silicon carbide with a large specific surface, to less equal to 100 m2 per gram, obtained by reaction of active carbon with high specific surface and silicon monoxide, according to claim tion 1 of the main patent application, to the implementation of reactions catalytic at high temperature, above 5000C and can be between 650 and 8000C, such as catalyst supports intended for internal combustion engine exhaust pipes, and the controlled oxidation of low molecular weight hydrocarbons, especially methane, in higher molecular weight hydrocarbons and longer carbon chain. 2. Application of silicon carbide according to claim 1, characterized in that the active carbon with a large specific surface is previously doped by impregnation with a solution of a polyvalent metal salt chosen from uranium, cerium, titanium, zirconium, hafnium and lanthanides.