Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
1999-12-30
2003-03-04
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C502S062000, C502S064000, C502S065000, C502S066000, C502S071000, C502S073000, C502S074000, C502S077000, C502S079000
Reexamination Certificate
active
06528031
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for preparing a noble metal-supported zeolite catalyst for selective reduction of nitrogen oxides exhausted under excess oxygen conditions. In particular, the present invention relates to a method for preparing a noble metal-supported zeolite catalyst effective for catalytic reduction of nitrogen oxides with methane, which comprises of filling the pores of zeolite with organic compounds prior to supporting active noble metal catalyst components on the zeolite. Through this preparation method, it is possible to locate noble metal components, which is essential for the designing of highly active catalysts for reduction of nitrogen oxides, precisely on the desired positions of zeolite pores.
2. Description of the Prior Art
Since Armor et al reported that nitrogen oxides (NOx) could be selectively reduced over a cobalt ion-exchanged Co-ZSM-5 catalyst by using methane as a reducing agent (Y. Li, and J. Armor,
Appl. Catal
. B 1 (1992) L31), it was recognized that the activation temperature of hydrocarbons is closely related with the temperature window and the activity of the selective reduction (SCR) of NOx. Accordingly, the research to develop noble metal-supported catalysts, for example platinum or palladium-supported catalysts having a high methane oxidation activity under a lean-bum condition for the SCR of nitrogen oxides has been actively carried out (Y. Nishizaka, and M. Misono,
Chemistry Letter
, 2237 (1994); J. H. Lee, and D. L. Trimm,
Fuel Processing Tech
., 42 (1995) 339). The said noble metal catalysts are supported on a support such as silica, zirconia, titanium and zeolite but their catalytic performance depends significantly on the types of supports. The catalysts supported on a non-microporous support such as alumina, silica, zirconia and titanium exhibit poor activity in excess oxygen atmosphere where oxygen content is 30% or more, while the catalysts supported on a zeolite having regular size of micropores maintain high SCR activity of NOx in excess oxygen atmosphere (R. Burch, and A. Ramli,
Appl. Catal
. B, 15 (1998) 49). Accordingly, it is thought that such noble metal-supported zeolite catalysts will become more and more important for the treatment of the exhaust gases from fixed sources excess oxygen such as gas turbines or boilers, or lean-bum engines.
A number of NOx-SCR catalysts wherein catalytic active noble metals such as platinum, palladium and rhodium supported on a zeolite and processes employing said catalysts have been suggested. Kanesaka et al. of Nissan Motor Co., Ltd., disclosed a catalyst comprising first layer of mononith coated with platinum, palladium or rhodium catalyst supported on alumina and second layer coated with copper or cobalt ion-exchanged on ZSM-5, mordenite or ferrierite, which showed excellent catalytic performance for the treatment of exhaust gas from lean-burn engines (U.S. Pat. No. 5,427,989 (1995)). Abe et al. of NGK Insulator, Ltd., tried to increase thermal stability of the noble metal supported zeolite catalysts by mixing them with alumina, titania, zirconia or silica (U.S. Pat. No. 5,164,350 (1992)). In addition, Oshima et al. of Toyota Jidosha Kabushiki Kaisha disclosed a NOx-SCR process based on Pt ion-exchanged zeolite catalyst working in internal combustion engines at the temperature range of 100 to 150° C., in which hydrogen was used as a reductant produced from methanol over a Cu-Ni-Cr/alumina reforming catalyt (U.S. Pat. No. 5,412,946 (1995)). Recently, Gardner et al. of Low Emissions Technologies Research and Development Partnership proposed a catalyst comprising a metal hydrate support such as titanium and zirconium doped with platinum, palladium, or a combination of these working in high oxidizing atmosphere and when the said catalyst was modified with an alkali or an alkaline earth metals further improvement brought in catalytic activity (U.S. Pat. No. 5,830,421, (1998)). Hepburn et al. of Ford Global Technology improved NOx-SCR activity by adding a NOx trapping material to a SCR catalyst comprising of Co, Cu, Pt, Au or Ag loaded on a zeolite or a heat resistant oxide. That is, Hepburn et al. succeeded in improving the NOx reduction performance of the catalyst by providing a noble metal-supported porous material to contact with exhaust gas for absorbing NOx prior to be reduced by the reductant (U.S. Pat. No. 5,727,385 (1998)).
As discussed above, since the noble metal supported on non-microporous supports are too simple in their catalytic functions to reduce selectively the NOx in excess oxygen condition, there have been an effort to design hybrid type catalysts comprising of a hydrocarbon oxidation site such as highly dispersed noble metal and a NOx reduction sites such as metal ion-exchanged micorporous zeolite through the introduction of adsorption capability of NOx. The said approaches contributed to increase catalytic activity at low temperature but there still remains further improvement lo bring sulfur resistance to the catalyst.
THE INVENTION
The objective of this invention is to provide a preparation method of NOx-SCR catalyst of which catalytic activity has increased by two times or more compared to that of conventional catalysts. In the present invention, in order to load noble metal components on a zeolite support, a new supporting method has been adopted different from simple impregnation or ion-exchange methods employed in the prior conventional methods. When the noble metals are supported on a zeolite according to the present invention, the expensive noble metals can be highly and precisely dispersed around the microporous zeolite compared to conventional methods. The prepared supported catalysts exhibit excellent NOx reduction activity more than two tunes higher than that of conventional catalysts especially in excess oxygen atmosphere.
The term “NOx reduction catalysts” means the catalysts capable of selectively reducing nitrogen oxides by the use of natural gas as a reducing agent in the presence of excess oxygen. The present invention is based on the concept of bifunctional catalyst proposed by Sang-Eon Park and Misono et al. Namely, the catalyst of the present invention is based on the result that the co-existence of two catalytic components capable of oxidizing hydrocarbons and reducing nitrogen oxides through the interaction with these activated hydrocarbons brought synergistic effect in catalytic activity, thereby making it possible to provide an efficient catalyst for selective reduction of nitrogen oxides (S.-E. Park,
React. Kinef. Catal. Lett
., 57 (1996) 339; J.-Y. Yan, H. H. Kung, W. M. H. Sachtler and M. C. Kung,
J. Catal
., 175 (1998) 294; C. Descorme, P. Gelin, C, leuyer and M. Primet.
J. Catal
., 177 (1998) 352.). Several types of preparation methods have been proposed to combine these two catalytic functions into one. The first is a method of physically mixing a noble metal component having excellent oxidizing activity with ion-exchanged zeolite catalysts having high NOx reduction capability. The second is a method of supporting an excess amount of noble metal catalyst component to a support (Korean patent application No. 96-956; M. Misono,
Cattech
. June (1998) 53). However, these two methods are not efficient to design a low temperature NOx-SCR catalyst working in excess oxygen condition.
Therefore, the objective of present invention is to provide a preparation method of NOx-SCR catalyst working with methane reductant at low temperature of 400° C. or less.
The other objectives and features of the present invention will become apparent to those skilled in the art from the following detailed description. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention are given by way of illustration and not limitation. Many changes and modifications within the scope of the present invention may be made without departing from the spirit thereof, and the invention includes all such modificati
Chang Jong San
Cho Jung Kuk
Lee Chul Wee
Lee Jin Woo
Park Sang Eon
Ildebrando Christina
Korea Research Institute of Chemical Technology
Silverman Stanley S.
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