Gas separation: processes – Solid sorption – Inorganic gas or liquid particle sorbed
Reexamination Certificate
2000-04-17
2002-04-30
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Inorganic gas or liquid particle sorbed
C095S900000, C096S108000, C423S239100, C502S400000
Reexamination Certificate
active
06379432
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a compound and a method for the selective absorption of NO, nitrogen oxides from gaseous mixtures containing carbon dioxide.
In particular, it relates to the absorption of nitrogen oxides from the exhaust gas of internal-combustion engines.
BACKGROUND ART
The literature (M. Machida et al.—J. Chem. Soc., Chem. Commun. (1990), p. 1165, and New Frontiers in Catalysis, Proc. of the 10th Intern. Congress on Catalysis, Budapest, Hungary, Elsevier (1993) p. 2644) describes mixed barium-copper oxides which are given the formula BaCuO
x
, where x has the values of 2.1 and 25, and are capable of reversibly absorbing nitrogen oxides by working within a certain temperature range, fixing them as barium nitrites and nitrates, and of releasing them by heating to temperatures higher than the absorption values, restoring the structure of the initial oxides.
The above mentioned mixed oxides are highly reactive also to carbon dioxide, which they fix as highly stable barium carbonate which, by depositing on the surface of the material, inhibits its further absorbing capability.
High reactivity to carbon dioxide therefore prevents use of compounds BaCuO
x
to absorb nitrogen oxides from mixtures rich in carbon dioxide, such as the exhaust gas of motor vehicles.
An attempt has been made to obviate this drawback by using mixtures of BaCuO
2.1
/MnO
2
which are scarcely sensitive to carbonatation.
Finally, it has been found that BaCuO
x
compounds tend to lose, over time, their capability of absorbing nitrogen oxides.
Application EP-A-666 102 describes the use of substances for adsorbing nitrogen oxides from the exhaust gas of engines designed to work with an excess of oxygen in the air/gasoline mix, capable of adsorbing NO and of converting it into NO
2
by virtue of the action of the excess oxygen that is present in the mix.
When the engine runs with an oxygen deficit (air/gasoline mix rich in gasoline), the adsorbed nitrogen dioxide reacts with the reducing gases that are present in the mix (CO and unburnt hydrocarbons), becoming N
2
and oxidizing the reducing gases to CO
2
and H
2
O.
The adsorbers used in the European application are essentially constituted by mixtures of barium carbonate and copper oxide formed locally during preparation by decomposition of copper nitrate and barium acetate with Ba/Cu ratios within broad ranges (from 1:3 to 3:1).
Said adsorbers, however, are entirely inactive in fixing nitrogen oxides in the absence of oxygen or in case of oxygen deficit, such as when the engine, at startup, runs with gasoline-rich air/gasoline mixes.
Furthermore, the temperature window in which the adsorbers are active is shifted toward high temperatures, thus preventing adsorption when the engine is running cold.
WO 97/28884 discloses a compound of formula Ba
2
Cu
3
O
6
suitable for adsorbing gases, among others, carbon dioxide.
U.S. Pat. No. 5,238,913 reports that compounds of formula Ba
2
Cu
3
O
5+x
(OL X L1) are suitable for preparing superconducting microcircuits. No indications are given about the method of preparation of the compounds and, in particular no mention is made of the compound Ba
2
Cu
3
O
6
.
DISCLOSURE OF THE INVENTION
It has now been unexpectedly found that the compound having the formula Ba
2
Cu
3
O
6
and the Raman spectrum characteristics as set forth in the claims is capable of selectively absorbing nitrogen oxides NO
x
from gaseous mixtures rich in carbon dioxide, possibly containing pollutants such as CO, SO
2
, hydrocarbons and mixtures thereof. Absorption occurs at temperatures between approximately 180° C. and 480° C., working at atmospheric pressure.
It has furthermore been found, and it is another aspect of the invention, that nitrogen oxide absorption kinetics is accelerated considerably by the presence of water vapor in the mixtures. In the case of NO
2
, the presence of oxygen and moisture shifts the absorption toward relatively low temperatures comprised between approximately 180° C. and ambient temperature. Preferably, NO
2
absorption is performed at temperatures above 35° C.-40° C.
By effect of the absorption of considerable amounts of NO
x
oxides, the compound of the invention decomposes forming barium nitrite and mono- and divalent copper oxides, if they are exposed to NO in the absence of oxygen, barium nitrate and bivalent copper oxide, if they are exposed to NO
2
or NO in the presence of oxygen.
The thermogravimetric curves plotted in
FIGS. 1 and 2
show the absorption of NO and NO2 as a function of the temperature (absorption of mixtures of 25% NO and 3% O
2
in helium, with a space velocity of 3000/h and 2.5% NO
2
and 2% O
2
in helium
itrogen with a space velocity of 3000/h and a heating rate of 20° C./min (percentages expressed by volume)).
By heating to temperatures above approximately 480° C., the compounds that have formed begin to decompose, releasing the nitrogen oxides and restoring the Ba
2
Cu
3
O
6
structure of the starting compound.
At temperatures above 480° C., barium nitrite and nitrate and copper oxide begin to react with each other, forming the compound Ba
2
Cu
3
O
6
and releasing, respectively, NO and NO
2
and possibly oxygen. In the range between 480° and 700° C., Ba
2
Cu
3
O
6
coexists alongside with barium nitrite and nitrate and with copper oxide; the Ba
2
Cu
3
O
6
fraction increases with time and temperature.
The selectivity of the Ba
2
Cu
3
O
6
with respect to CO
2
depends considerably on the preparation method.
It has been found, and it is another aspect of the invention, that the compound of the invention considerably increases its resistance to carbonatation if it is prepared starting from barium nitrate and copper oxide intimately mixed in a cationic ratio of 2:3, subsequently heating the mixture to 640° C.-650° C. in an air stream until the barium nitrate is completely decomposed and then cooling the mixture in air stream at a rate of no more than 20° C./min.
The air can be replaced with oxygen
itrogen mixtures or oxygen/inert gas mixtures containing up to 25 g/m
3
of water vapor and up to 400 ppm of CO
2
.
It has furthermore been found that the presence of nitrogen oxides during the cooling of the material, or their addition to the reaction atmosphere to complete the synthesis, facilitate the formation of the carbonatation-resistant materials.
The curve of carbonatation as a function of temperature which is typical of the compound Ba
2
Cu
3
O
6
thus prepared as above specified is reported in
FIG. 3
(stream of 10% CO
2
, 10% H
2
O, complement with mixtures of nitrogen and argon, exposure 5 hours, percentages by volume).
For comparison, the circles indicate the carbonatation behaviour of a non-resistant compound BaCuO
2.5
prepared according to the methods described in literature.
The carbonatation curve of the compound supported on alumina is similar to the curve of the above mentioned compound. The preparation is made by immersing porous aluminum oxide, dehydrated beforehand, in a near-saturated solution of barium nitrate and copper nitrate in deionized water, using a barium ion/copper ion ratio of 2:3 and working at temperatures between 20° C. and 80° C.
The material, impregnated with the solution, is dried at 110° C.-150° C. and then subjected to the above described heat treatment (reaction at 640° C.-650° C. and then cooling at a rate of no more than 20° C./min).
The procedure can be repeated in order to increase the filling of the pores of the aluminum oxide until saturation is reached.
Approximately 3.5% by weight of supported compound is obtained for each impregnation/heat treatment cycle.
The curve of
FIG. 3
shows that the compound Ba
2
Cu
3
O
6
prepared as mentioned above is not sensitive to carbonatation up to approximately 420° C. (less than 0.4% increase in weight after 5 h of exposure). The increase is less than 2% at 500 ° C., again after 5 h of exposure.
Resistance to carbonatation decreases considerably if the compound Ba
2
Cu
3
O
6
is prepared at 800° C. and then cooled quickly to ambient temperature (rate of approximately 5°
Calestani Gianluca
Matacotta Francesco Cino
Consiglio Nazionale Delle Ricerche
Josif Albert
Modiano Guido
O'Bryne Daniel
Spitzer Robert H.
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