Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
1999-10-12
2001-04-17
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C423S235000
Reexamination Certificate
active
06217838
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a process for eliminating oxides of nitrogen (NO and N
2
, known as NO
x
) in a medium which is super-stoichiormtric in oxidising agents.
This process relates to eliminating oxides of nitrogen present in exhaust gases from automotive or stationary lean-burn engines, whether lean burn spark ignition engines or compression ignition engines. This process can also be used to eliminate oxides of nitrogen from fumes from power station, from waste incinerators or from gas turbines. Such gases are characterized by oxides of nitrogen contents of several tens to several thousands of parts per million (ppm), by comparable amounts of reducing agents (CO, H
2
, hydrocarbons) but above all by high concentrations of oxygen (from 1% to close to 20% by volume.)
The high toxicity of oxides of nitrogen and their role in the formation of acid rain and tropospheric zone have led to the instigation of strict regulations limiting the discharge of such compounds. In order to satisfy those regulations, it is generally necessary to eliminate at least a portion of such oxides present in exhaust gases from automotive or stationairy engines, from turbines or from power stations or incinerators.
In the case of engine exhaust gases, the elimination of oxides of nitrogen by thermal decomposition or preferably by cartalytic decomposition can be envisaged, but the high temperatures demanded by this reaction are incompatible with those of the exhaust gases. They can only be reduced using the reducing agents which are present, albeit in small quantities, in the exhaust gases (CO, H
2
, unburned hydrocarbons or where combustion in the engine has been imperfect), and also using complemnentary reducing compounds which would have to be injected upstream of the catalyst. Such reducing agents are hydrocarbons, alcohols, ethers or other oxygen-containing compounds.
The reducing agent can be the vehicle fuel. Such fuels can be gasolines, which may or may not have added oxygen-containing compounds (for example alcohols, ethers), gas oils, liquefied gases (LPG), or compressed natural gases (CNG).
The reduction of nitrogen monoxide and dioxide (NO and NO
2
) can, in accordance with the equations below, lead to the formation of either molecular nitrogen (N
2
), or nitrous oxide (N
2
O).
Molecular nitrogen (N
2
) is the desired inoffensive product while the formation of nitrous oxide (N
2
O), a non inert compound, should be avoided as far as possible.
The essential parameters for characterising the performances of the catalyst for eliminating oxides of nitrogen are:
the temperature at which the reaction is maximal;
the conversion or degree of elimination of NO
x
;
the selectivities for N
2
and N
2
O;
and in particular the N
2
yield of the NO
x
reduction reaction which can be expressed by the relationship:
N
2
yield
=
100
×
NO
x
disappeared
-
2
×
(
N
2
O
formed
)
NO
x
initial
PRIOR ART
The majority of work on developing catalysts which are ative in eliminating oxides of nitrogen in an oxidising medium has focussed on the use of transition metals (generally copper) exchanged on zeolites with Si/Al atomic ratios which are generally in the range 3 to 100 and in which the structures can be of different types (mordenite, faujasite, ZSM5, etc.) (United States patent U.S. Pat. No. 5,149,512). The thermal stability of copper catalysts exchanged on zeolitc can be improved by adding alkaline-earth and rare earth elements (lanthanum, P. Budi et al., Catalysis Letters, 41 (1996) 47-53, and cerine, Y. Zhang et al., Journal of Catalysis 194 (1996), 131-154). It should, however, he noted that these catalysts are highly selective for total reduction of oxides of nitrogen to molecular nitrogen (FR-A-2 655 565 and U.S. Pat. No. 5,149,511). These catalysts are active in converting oxides of nitrogen for temperatures in the range 350° C. to 550° C.
A large amount of work has been carried out on alumina type supports. Alumina has the advantage of being stable under diesel exhaust gas conditions. Alumina is active in reducing NO
x
to nitrogen by hydrocarbons at temperatures in the range 450° C. to 750° C. (Y. Kintaichi et al., Catalysis Let. 6 (1990) 239-244; N. Okazaki et al., Catalysis Let. 49 (1997) 169-174).
Catalysts based on transition metals exchanged on zeolite or of the alumina type are active in reducing NO
x
at temperatures of over 350° C. These conditions thus limit their use to depollute lcan burn gasoline engines and heavy diesel engines when these are functioning at full throttle and at high speeds. However, the exhaust gas temperature in light diesel engines is generally in the range 150° C. to 400° C. and rarely exceeds 500° C., which restricts the efficacy of such catalysts in eliminating oxides of nitrogen when the vehicle is used under normal conditions.
The use of precious metals as the active phase also enables a major portion of the oxides of nitrogen to be eliminated in proportions comparable to those measured with copper catalysts (EP-A-0 709 129). Such catalytic phases have the advantage of being active at much lower temperatures (200-300° C.), which is an essential advantage when depolluting exhaust gases from diesel vehicles where the engine exhaust temperatures are generally in the range 150° C. to 400° C.
Precious metals can be impregnated into different supports such as alumina, silica, zirconia, titanium oxide or zeolites. Platinum catalysts supported on zeolite (ZSM-5) can be prepared by ion exchange (EP-Al-0 709 129).
Platinum based catalysts am generally highly active at low temperatures for converting oxides of nitrogen NO and NO
2
. However, the majority of such catalysts have the disadvantage of only partially reducing the NO
x
, namely the major product formed is not molecular nitrogen (N
2
) but nitrous oxide (N
2
O). Further, supported platinum-based catalysts effect NO
x
reduction over a narrow temperature range. 180° C. to 350° C. (R. Burch et al., Catalysis Today 25 (1995) 185-206).
Certain catalytic metals such as indium or zinc can reduce NO
2
to nitrogen by hydrocarbons while on those metals, the reduction of NO to nitrogen by hydrocarbons is very low. M. Iwamoto et al., (Chemistry Letters (1997) 1283-1284; J. Chem. Soc., Chem. Commun. (1997) 37-38) propose tho reduction of NO
x
using two distinct catalysts. The first platinum based catalyst can oxidise NO to NO
2
. The second Zn/MFI zeolite or In/MFI zeolite type catalyst, placed downstream of the oxidation catalyst, reduces NO
2
to N
2
. The reducing agent is introduced into the gas stream between the two catalytic beds. That process reduces NO
x
between 200° C. and 500° C. in the presence of an excess of oxygen and in the absence of water. However, the conversion activity for NO
x
of that process is greatly diminished by the presence of water, with a gas stream which represents a diesel exhaust gas.
DESCRIPTION OF THE INVENTION
The present invention provides a process for reducing emissions of oxides of nitrogen in a medium which is super-stoichiometric in oxidising agents, comprising:
a) a step for oxidising at least a portion of the oxides of nitrogen in the presence of an oxidation material;
b) a step for injecting organic compounds the molecules of which comprise at least one atom selected from carbon, hydrogen, oxygen and nitrogen;
c) a step for adsorbing at least a portion of said organic compounds onto an adsorption material in the form of molecular species and/or carbonaceous residues;
d) a step for elective reduction of at least a portion of the oxides of nitrogen to molecular nitrogen by at least a portion of the molecular species and/or carbonaceous residues formed on the adsorption material.
The present invention provides a process for reducing NO
x
which can surprisingly obtain a comparable if not superior efficacy to that of prior art formulations, to eliminate oxides of nitrogen, from low temperatures and over a very broad temperature range (150-550° C.), in a highly oxidising gaseous mi
Bouchez Mathias
Bourges Patrick
Mabilon Gil
Martin Brigitte
Griffin Steven P.
Institut Francais du Pe'trole
Millen White Zelano & Branigan
Strickland Jonas N.
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