Abstract:
A catalyst for purifying exhaust gas in vehicles may include a precious metal and porous structures that serve as a supporting material for the precious metal. The porous structures are comprised of a plurality of channels which are connected with each other by a plurality of bridges. The channels may have multiple entrances that allow reactants to pass through and react with the precious metal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority of Korean Patent Application Number 10-2010-0120937 filed in the Korean Intellectual Property Office on Nov. 30, 2010, the entire contents of which application is incorporated herein for all purposes by this reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a catalyst using a precious metal. More particularly, the present invention relates to a highly efficient catalyst using a three-dimensional porous structure as a supporting material for the precious metal in order to prevent an inaccessible region of the precious metal from being generated, and further improve the diffusion of exhaust gas. 
     2. Description of Related Art 
     Recently, according to the increasing usage of vehicles and severe traffic, air pollution by exhaust gas is becoming an issue. In order to regulate exhaust gas and enforce the regulations, many countries have established emission standards for pollution substances such as carbon monoxide (CO), hydrocarbon (HC), and nitrogen oxide (NOx) in exhaust gas. 
     Usually, catalysts coated with precious metals such as platinum (Pt), palladium (Pd), and rhodium (Rh) are used for removing noxious substances from the exhaust gas of vehicles. These catalysts remove the noxious substances from the exhaust gas and purify the exhaust gas by promoting decomposition of the hydrocarbons, oxidization of the carbon monoxide, and reduction of the nitrogen oxide. 
     A catalyst uniformly coated with a precious metal in a supporting material as a purifying catalyst of exhaust gas has been published. In a case of a conventional catalyst for purifying vehicle exhaust gas, the catalyst is manufactured by coating catalytic substances including the expensive precious metal on a supporting material shaped as a honeycomb in order to increase the contact area between the catalyst and the exhaust gas, and thus to increase the reaction area. 
     Typically, square cells are used as the unit cells of the honeycomb supporting material. However, as shown in  FIG. 1 , using square cells in a catalyst creates corners where certain amount of catalytic substances accumulates resulting in a thicker catalytic layer  14  in the vicinity of the corners. 
     Noxious reactants  16  such as CO, HC, and NOx in the exhaust gas diffuse into the catalytic layer and are then converted into harmless substances in contact with the precious metal (Pt, Pd, and Rh). The arrow in  FIG. 1  shows the diffusion of the exhaust gas around a corner. Because the catalytic layer there is thicker, the exhaust gas cannot diffuse into the region  15  of the precious metal. As such, the catalyst in deep corners becomes an inaccessible region, that is, a dead zone into which the CO, HC, and NOx cannot diffuse. So the precious metal in the inaccessible region  15  cannot participate in reaction. 
     To solve the above problems, a hexagonal cell  20  in  FIG. 2  was contrived. But the isotropic strength of the hexagonal cell  20  is weaker than that of the square cell  10 , so few hexagonal cells  20  are actually used. 
       FIG. 3(A)  shows a schematic view of a diffusion path of reactants in a unit cell where  12  refers to a cell wall and can be made of cordierite.  FIG. 3(B)  and  FIG. 3(C)  show schematic views of two catalysts where the one shown in  FIG. 3(C)  is deteriorate. In the past, an amorphous powder was used as a supporting body  50 , so the density of the catalytic layer grew large, the diffusion of the exhaust gas was deteriorated thereby, and an inaccessible region of the precious metal  15  was generated. In addition, the reaction surface was reduced and pores were covered after the precious metal was sintered (referring to  40  and  50   a ) or after the precious metal was reacted with sulfur in exhaust gas and was poisoned or otherwise contaminated (referring to  55 ). Furthermore, inaccessible regions of precious metal  50   b  were generated. 
     The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art. 
     BRIEF SUMMARY 
     Various aspects of the present invention have been made in an effort to address the problems mentioned above and to provide a highly efficient catalyst by using porous structures as precious metal supporting materials to improve the diffusion of exhaust gas. Various aspects of the present invention provide catalysts to purify the exhaust gas from vehicles. The catalysts use a precious metal and a supporting material that includes the precious metal and has porous structures. 
     One aspect of the porous structures according to the present invention is characterized in that they have a plurality of channels and the channels are connected with other channels by bridges. 
     Another aspect of the porous structures according to the present invention is characterized in that the structures include channels through which reactant pass thereof. 
     Yet other aspects of the porous structures according to the present invention is characterized in that they are made of mesoporous nanoparticles such as MCM or SBA type materials. These mesoporous nanoparticles include mesoporous silica and mesoporous alumina. 
     Various aspects of the present invention provide several advantages. First, particles of precious metal can be uniformly spread so that the reaction surface can be increased. Secondly, even if one channel entrance of the structures is blocked by sintering of a supporting material poisoned or contaminated with sulfur, the exhaust gas can pass through other channel entrances so the catalyst can still be reachable and engaging in reaction. 
     The methods and apparatuses of the present invention have other features and advantages which will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated herein, and the following Detailed Description, which together serve to explain certain principles of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a unit cell including a conventional square supporting material. 
         FIG. 2  is a schematic view of the unit cell including a conventional hexagonal supporting material. 
         FIG. 3(A)  shows a schematic view of the diffusion path of reactants in a unit cell where number  12  refers to a cell wall. 
         FIG. 3(B)  is a schematic view of a catalyst. 
         FIG. 3(C)  is a schematic view of another catalyst 
         FIG. 4  is a perspective view of exemplary porous structures according to the present invention. 
     
    
    
     It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment. 
     In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing. 
     DETAILED DESCRIPTION 
     Reference will now be made in detail to various embodiments of the present invention(s), examples of which are illustrated in the accompanying drawings and described below. While the invention(s) will be described in conjunction with exemplary embodiments, it will be understood that present description is not intended to limit the invention(s) to those exemplary embodiments. On the contrary, the invention(s) is/are intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims. 
     Referring to  FIG. 4 , various embodiments of the present invention are configured to reduce the wasting of expensive precious metals by minimizing inaccessible regions of the precious metal  15 . As such the quantity of the precious metal required for making a catalyst is minimized. For this, a porous structure  100  is used so that the dead zone, that is, the inaccessible region of the precious metal  15 , can be minimized by improving the diffusion of the exhaust gas. In various embodiments of the present invention, porous structures  100  have a plurality of channels and are used as a supporting material of the precious metal. The porous structures  100  are made of mesoporous nanoparticles such as MCM or SBA type materials. 
     The most common types of mesoporous nanoparticles are MCM-41 and SBA-15. MCM-41 among the MCM series is made by using a micro-colloidal crystal template method and synthesizing well-ordered macroporous silica. SBA-15 among the SBA series is made by synthesizing polymer-globular particles using an emulsion polymerization method as an example. Currently, the synthesized polymer-globular particles are arranged regularly by self-assembling and are blended with a mixture, which is a mother liquid, of a hydrochloric acid solution and a triblock copolymer, and then templates of uniform porous globular particles can be obtained and finally the templates are dried and calcinated. The SBA-15 used in various embodiments of the present invention has a mesoporous structure made of silica, and it has pores of 5 to 50 nm that are hydrothermally stable by using an amphiphilic block hollow polymer. For a detailed description of the manufacturing method thereof, one can consult pertinent publications. 
     For the purpose of reference, micro, macro, and meso in this specification are defined according to IUPAC definition (International Union of Pure and Applied Chemistry) as below 2 nm, over 50 nm, and 2 nm to 50 nm in pore size respectively. In various embodiments of the present invention, mesoporous silica and mesoporous alumina may be used. 
     Hereinafter, the porous structures  100  will be described in greater detail. 
     The porous structures  100 , as shown in  FIG. 4 , have a plurality of channels  110  and the channels  110  are connected with each other by bridges  120 . The channels  110  have entrances through which the exhaust gas can pass. and the bridges  120  are constructed in such a way that small precious metal powders can rarely move through. 
     If the catalyst using the above structures  100  as a supporting material is mounted on a vehicle, the particles of the precious metal are agglomerated and sintered, so that the required reaction surface is reduced and the decrease of activity can be prevented. In addition, even when an entrance of a channel  110  is blocked in sintering a supporting material poisoned or contaminated with sulfur, gaseous reactants can still pass through other entrances of the channel  110  and remain active by preventing the entrance of the pores from being blocked and by preventing the precious metal from becoming an inaccessible state. Furthermore, the precious metals are uniformly dispersed as represented by  50   c  in  FIG. 4 . As such, the reaction surface is increased and the activity of the catalyst is enhanced. 
     In other words, a plurality of channels  110  in various embodiments of the present invention are connected with each other, the gaseous reactants can pass through the entrances of the channels  110 , and the diffusion of exhaust gas can be improved thereby. In addition, the usage of the precious metal can be minimized to an optimal quantity. 
     The foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teachings. The exemplary embodiments were chosen and described in order to explain certain principles of the invention and their practical application, to thereby enable others skilled in the art to make and utilize various exemplary embodiments of the present invention, as well as various alternatives and modifications thereof. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.