Adsorbent catalyst

Details Adsorbent catalyst Adsorbent catalyst

2001-04-10

2004-11-16

Bos, Steven (Department: 1754)

Catalyst, solid sorbent, or support therefor: product or process

Zeolite or clay, including gallium analogs

And group iii or rare earth metal or lanthanide containing

C502S303000, C502S304000, C502S328000, C502S334000, C502S339000, C502S241000, C502S242000, C502S243000, C502S246000, C502S250000, C502S251000, C502S252000, C502S262000, C502S263000

Reexamination Certificate

active

06818582

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to an adsorbent catalyst for purifying exhaust gases or combustion gases, especially exhaust gases from gasoline, diesel or natural gas engines.
BACKGROUND OF THE INVENTION
With gasoline engines, significant decreases (10-20%) in fuel consumption have been attained with new engines where direct injection of gasoline and excess air are used. Depending on running stage, the blend ratio air/fuel (A/F=air/fuel) is in these engines clearly on the dilute side (&lgr; is between 1.1 and 2.4) for extended periods, whereby air is introduced in excess in relation to the amount needed for complete combustion of the fuel. At other times, the blend ratio of the engine is stoichiometric (&lgr;=1) or rich (&lgr;<1), whereby these circumstances must normally be adapted to in varying driving conditions and by high driving speeds. The management system (EMS=Engine Management System) strives to comply with the running parameters programmed in the memories of the vehicle that are based on the engine and driving condition mapping of the automobile maker. A diesel engine is traditionally more fuel conscious especially in heavy motor vehicles, and the injection technique of the fuel has developed together with the engine management systems to the point, where raw emissions and especially the amount of emitted particles are minor. The increasing change-over to dilute blend engines has, however, made the functioning of the traditional removal techniques of the nitrogen oxide exhausts more difficult.
With a three-function catalyst the concentrations of nitrogen oxides, carbon monoxide and hydrocarbons may be decreased by over 90% efficiency during normal motor vehicle test cycles (according to Euro 3/Euro 4 levels). When using a three function catalyst, one strives to keep the circumstances (air/fuel blend ratio, A/F) as stoichiometric as possible with the aid of responses from the &lgr; sensors and the engine management system. The emission limits for particles and for NO
x
become stricter when shifting from Euro 3 to Euro 4: particles down to 0.025 g/km and No
x
down to 0.25 g/km.
However, in exhaust gases containing oxygen in excess, the amounts of reducing agents and their selectivity towards reducing NO
x
are low. Normally, gaseous emissions (HC, CO) are purified at more than 80% conversion, and particles at about from 20% to 40% conversion when using an able oxidizing catalyst and &lgr; being clearly over 1. The conversion of nitrogen oxides is in diesel exhausts normally about from 0% to 20%, whereby even a four-function catalyst is mentioned (CO, HC, NOx, particles). In diesel oxidizing catalysts, normally Pt is used as the active metal, it being resistant to SO
x
. The catalysts are often positioned as close to the engine as possible (CC=Closed Coupled), instead of being positioned under the body (UF=Under Floor), as usual.
In order to decrease the amount of nitrogen oxides, Ir catalysts, used together with three-function catalysts or Pt catalysts, have been developed and used commercially even in Euro 2 level direct injection automobiles (GDI=Gasoline Direct Injection). However, this kind of catalyst constantly requires a relatively high hydrocarbon content in the exhaust gas, the thermal and chemical resistance of the Ir catalysts is rather poor (max 800-900° C.), and the NO
x
conversion remains rather low. Even Pt—Ir—Rh/ZSM5 catalysts have previously been used in dilute gasoline subjects (Iwakuni et al., Science and Tech. in Catal. 1998). Although one has strived to increase the endurance of zeolite based catalysts, it is, however, insufficient in full load driving circumstances.
Because catalysts based on continuous NO
x
reduction do not function at the levels required by Euro3 or Euro4 exhaust limits, new types of catalyst have been developed that are based on NO
x
adsorption (NO
x
trap) (SAE Publication 982593). Their functioning is based on adsorbing nitrogen oxides during dilute phases into the NO
x
trap components of the catalyst, one of which is e.g. BaO. Nitrogen oxides are oxidized to NO
2
, they are adsorbed as nitrates (Ba(NO
3
)
2
) in the dilute phase (duration, e.g. from 15 s to 5 min) and they are reduced to nitrogen in the short enrichment period arranged on purpose (duration, e.g. from 1 s to 5 s, A/F ratio less than stoichiometric). The method may be tuned (timing, regenerations) to function in accordance with the engine and the driving conditions. The most usual adsorbent Ba binds the sulfur oxides (SO
x
) of the exhaust gas tightly as sulfates on its surface, and this weakens functioning of the catalyst as time goes on. Depending on the sulfur content of the fuel, the catalyst must be totally regenerated every 1000-6000 km to rid it of sulfate. Barium (Ba) sulfate can be decomposed by raising the temperature of the exhaust gas to more than 650° C. for a time long enough in clearly rich circumstances. Thereby the sulfates are decomposed as sulfides, and are liberated mainly as hydrogen sulfide or COS into to exiting exhaust gas. Thus the functioning of the catalyst is partly or totally recovered. In unfavorable circumstances, BaO may bind CO
2
as carbonates on its surface more tightly than nitrates. The functioning of catalysts based on Ba calls for a very low sulfur level of the fuel, and also, the level of functioning of the catalysts mentioned is not high enough.
The exhaust gas flows continuously through the NO
x
trap catalyst, the continued functioning of which is based on the enrichments guided by the management system (EP 0 560 991, 1993). HC and CO are presented as functioning as reducing agents in the enrichments and the dilute phases are of a duration more than 50 times longer than the enrichment phases. The NO
x
adsorbent in the catalyst is K, Na, Li, Cs, Ba, Ca, La or Y and as noble metal, Pt. Also a Ba—Cu composite is claimed to function. After the NO
x
trap catalyst, or before it, there may be a normal three-function catalyst. In the EP patent, even a system for purifying diesel engine exhausts with the same catalyst is described.
Another concept (EP 0 838 255) contains on the surface of a porous support (aluminum oxide) alkali metal (Na, Li, K, Rb), Ti and a noble metal (Rh, Pt, Pd). Also, the catalyst may contains rare earth metals or Mg. It has been claimed that by very high Na and K contents (over 30% by weight, EP 0 857 510) or Li contents (over 10% by weight, WO 97/47 373) together with noble metals, an efficient NO
x
trap material has been accomplished. Sr (6-15% by weight) may be used together with Zr and sulfate even in place of Ba in the catalyst (U.S. Pat. No. 5,753,192).
With various mixed oxides (in general A
a
B
b
O
4
, where B is mainly Al, and A is Mg, Ca, Mn, Fe, Ni, Co, Cu, Zn, Sn or Ti) there have been accomplished NO
x
adsorption capacities (WO 98/56 492).
Use of W-phosphate in the Pt catalyst has been proposed in order to hinder the harmful effects of sulfur in NO
x
adsorption (EP 0 864 353).
A solution has been proposed, according to which there are two NO
x
trap catalysts side-by-side whereby concomitantly one is at the adsorptive stage and the other at the reductive stage (WO 98/45 582). The catalyst contains Sr, Ba, Ca, BaTiO
3
, BaZrO
3
, LaZrO
3
, MnO
2
, LaMNO
x
and a mixture of oxides of La, Ce, Ti and Zr, as well as Pt, Rh or Pd. Similar compounds (BaMnO
3
, CaMnO
3
) have been used also in another Patent (U.S. Pat. No. 5,906,958). The method is complicated and it requires a double amount of the catalyst as well as a three-way valve that functions in hot circumstances (even 900° C.), in order to direct the exhaust gas.
It has been claimed that a mixture of Ti/mordenite and mixed oxide (La, Ba, Co) functions in the presence of hydrocarbons, giving a good NO
x
conversion (DE 44 45 945).
There has been presented a two-layer catalyst where the Pt and NO
x
adsorbents (Ba, K) are in the first layer and the Rh is in the second layer (WO 99/00 177). The aim is to oxidize NO to NO
2
in the Pt layer and to reduce the nitrogen oxi

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