SYNTHESIS OF METAL OXIDE CATALYSTS USING SUPERCRITICAL CARBON DIOXIDE EXTRACTION

Abstract

A metal oxide catalyst synthesized using supercritical carbon dioxide extraction is provided, wherein the metal oxide catalyst includes an active site containing at least one type of metal oxide and a support for loading the active site and the metal oxide is an oxide of a metal selected from the group consisting of transition metals (atomic number 21 to 29, 39 to 47, 72 to 79, or 104 to 108), lanthanide (atomic number 57 to 71), post-transition metals (atomic number 13, 30 to 31, 48 to 50, 80 to 84, and 112), and metalloids (atomic number 14, 32 to 33, 51 to 52, and 85) in the periodic table, and a combination thereof.

Claims

1. A method of synthesizing a metal oxide catalyst, which comprises an active site containing at least one type of metal oxide and a support for loading the active site, the method using supercritical carbon dioxide (CO.sub.2) extraction.

2. The method of claim 1, comprising: precipitating a precursor of metal oxide catalyst crystalline grains onto a surface of a support after dissolving the precursor of the metal oxide catalyst crystalline grains in a synthetic solvent; drying a catalyst crystalline grain precursor-support intermediate product using supercritical CO.sub.2 extraction; and calcining the dried catalyst crystalline grain precursor-support intermediate product to synthesize a metal oxide catalyst.

3. The method of claim 1, wherein the supercritical CO.sub.2 extraction is performed at a temperature ranging from 50 to 150° C., for 0.1 to 24 hours, at a flow rate ranging from 10.sup.−5 to 10.sup.5 mL min.sup.−1, and at a CO.sub.2 pressure ranging from 75 to 150 atm.

4. The method of claim 2, wherein a supercritical CO.sub.2 fluid extracted by the supercritical CO.sub.2 extraction weakens an interaction between the support and the synthetic solvent.

5. A metal oxide catalyst comprising: an active site containing at least one type of metal oxide; and a support onto which the active site is loaded, wherein the metal oxide is an oxide of a metal selected from the group consisting of transition metals (atomic number 21 to 29, 39 to 47, 72 to 79, or 104 to 108), lanthanide (atomic number 57 to 71), post-transition metals (atomic number 13, 30 to 31, 48 to 50, 80 to 84, and 112), and metalloids (atomic number 14, 32 to 33, 51 to 52, and 85) in the periodic table, and a combination thereof.

6. The metal oxide catalyst of claim 5, wherein the active site is porous and has a diameter range of 0.1 nm to 500 μm.

7. The metal oxide catalyst of claim 5, wherein the active site has a composition range of 10.sup.−4 to 50 parts by weight based on 100 parts by weight of the support.

8. The metal oxide catalyst of claim 5, wherein the support contains at least one element selected from the group consisting of alkaline earth metals (atomic number 4, 12, 20, 38, 56, and 88), transition metals (atomic number 21-29, 39-47, and 72-79, or 104-108), lanthanide (atomic number 57-71), post-transition metals (atomic number 13, 30-31, 48-50, 80-84, and 112), and metalloids (atomic number 14, 32-33, 51-52, and 85) in the periodic table, and carbon (C).

9. The metal oxide catalyst of claim 5, wherein the support has microporosity, mesoporisity, macroporosity, or hierarchical porosity.

10. The metal oxide catalyst of claim 8, wherein the support contains at least one oxide of the element.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1A to 1D show the results of scanning electron microscopic (SEM) observation (first line) and high resolution transmission electron microscopic (HRTEM) observation (second line) of catalysts synthesized in Embodiments 1 and 2 of the present invention.

[0024] FIG. 2 is a graph showing X-ray diffraction (XRD) patterns of the catalysts synthesized in Embodiments 1 and 2 of the present invention.

[0025] FIGS. 3A and 3B are graphs showing selected area electron diffraction (SAED) patterns of the catalysts synthesized in Embodiments 1 and 2 of the present invention.

[0026] FIGS. 4A and 4B are graphs showing H.sub.2-temperature programmed reduction (H.sub.2-TPR) profiles of the catalysts synthesized in Embodiments 1 and 2 of the present invention.

[0027] FIGS. 5A and 5B are graphs showing X-ray photoelectron (XP) spectra in the O 1s region of the catalysts synthesized in Embodiments 1 and 2 of the present invention.

[0028] FIGS. 6A and 6B are graphs showing a nitrogen oxide (NO.sub.X) conversion X.sub.NOX and nitrogen (N.sub.2) selectivity S.sub.N2 in a selective catalytic reduction (SCR) reaction of the catalysts synthesized in Embodiments 1 and 2.

[0029] FIG. 7 is a graph showing the performance change X.sub.NOX/X.sub.NOX,0 according to the presence or absence of oxygen (O.sub.2) in the SCR reaction of the catalysts synthesized in Embodiments 1 and 2.

[0030] FIGS. 8A and 8B illustrate graphs showing ammonia conversions X.sub.NH3 and N.sub.2/NO.sub.X/N.sub.2O selectivity S.sub.N2/S.sub.NOX/S.sub.N2O in a selective catalytic oxidation (SCO) reaction of the catalysts synthesized in Embodiments 1 and 2.

[0031] FIGS. 9A and 9B illustrate graphs showing performance changes X.sub.NH3/X.sub.NH3,0 and S.sub.N2 according to the presence or absence of O.sub.2 in the SCO reaction of the catalysts synthesized in Embodiments 1 and 2.

[0032] FIG. 10 is a graph showing long-term stability in the presence of 50 ppm of SO.sub.2 at low temperature (180° C.) in the SCR reaction of the catalysts synthesized in Embodiments 1 and 2.

[0033] FIG. 11 is a graph showing long-term stability in the presence of 500 ppm SO.sub.2 at low temperatures (180° C. and 200° C.) in the SCR reaction of catalysts synthesized in Embodiments 1 and 2.

[0034] Throughout the drawings and the detailed description, unless otherwise described, the same drawing reference numerals will be understood to refer to the same elements, features, and structures. The relative size and depiction of these elements may be exaggerated for clarity, illustration, and convenience.

DETAILED DESCRIPTION

[0035] The following detailed descriptions of the invention will be made with reference to the accompanying drawings illustrating specific embodiments of the invention by way of example. These embodiments will be described in detail such that the invention can be carried out by one of ordinary skill in the art. It should be understood that various embodiments of the invention are different, but are not necessarily mutually exclusive.

[0036] For example, a specific shape, structure, and characteristic of an embodiment described herein may be implemented in another embodiment without departing from the scope of the invention. In addition, it should be understood that a position or placement of each component in each disclosed embodiment may be changed without departing from the scope of the invention.

[0037] Accordingly, there is no intent to limit the invention to the following detailed descriptions. The scope of the invention is defined by the appended claims and encompasses all equivalents that fall within the scope of the appended claims. In the drawings, like reference numerals denote like functions, and the sizes of elements may be exaggerated for convenience of explanation.

[0038] Hereinafter, to allow one of ordinary skill in the art to easily carry out the invention, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0039] A metal oxide catalyst according to an embodiment of the present invention includes an active site corresponding to a region onto which a reactant is adsorbed and from which a product is detached after reaction, and a support for loading the active site thereon.

[0040] A method of synthesizing the catalyst composed of the above-described active site and support by using supercritical carbon dioxide (CO.sub.2) extraction includes 1) precipitating a precursor of metal oxide catalyst crystalline grains onto a surface of the support, 2) drying a catalyst crystalline grain precursor-support intermediate product using supercritical CO.sub.2 extraction (removing a synthetic solvent), and 3) calcining the dried catalyst crystalline grain precursor-support intermediate product to synthesize a metal oxide catalyst.

[0041] The aforementioned catalyst crystalline grain precursor-support intermediate product may be prepared by various methods. For example, the catalyst crystalline grain precursor-support intermediate product may be prepared by one or more of hydrothermal synthesis, solvent thermal synthesis, non-templated or templated synthesis, wet or dry impregnation with pH control, or thermal decomposition using metal complex. However, in order to maximize the advantages provided by the supercritical CO.sub.2 extraction described above/below, it is preferable to prepare an intermediate product in which a precursor of catalyst crystalline grains is precipitated onto a support.

[0042] The supercritical CO.sub.2 extraction for removing the synthetic solvent by drying the catalyst crystalline grain precursor-support intermediate product described above may be carried out by loading the precursor-support intermediate product on a batch type/continuous type reactor, thereafter exposing the surface of the intermediate product to a carbon dioxide processing gas under a predetermined flow rate/temperature/pressure, preferably at a temperature and pressure (31° C. or higher and 72.8 atm or higher) at which a supercritical carbon dioxide fluid is generated. Table 1 below shows a range of conditions for generating a supercritical carbon dioxide fluid.

TABLE-US-00001 TABLE 1 CO.sub.2 pressure Processing gas flow rate Exposure time Temperature (atm) (mL min.sup.−1) (h) (° C.) 75-150 10.sup.−5~10.sup.5 0.1~24 50-150

[0043] When supercritical carbon dioxide extraction is performed under the condition of a temperature of 50° C., 0.1 hours, a flow rate of 10.sup.−5 mL min.sup.−1, or a CO.sub.2 pressure of less than 75 atm, the effect of supercritical carbon dioxide extraction from the catalyst surface may be insignificant. On the other hand, when the supercritical carbon dioxide extraction is performed under the conditions of a temperature of 150° C., 24 hours, a flow rate of 105 mL min.sup.−1, or a CO.sub.2 pressure of greater than 150 atm, the structure of an active site/support may be damaged/deformed, surface labile oxygen species/oxygen vacancies may be eliminated, or redox properties may be severely deteriorated. Accordingly, the supercritical carbon dioxide extraction for removing the synthetic solvent included in the precursor-support intermediate product may be performed within the range of the above-described conditions.

[0044] The metal oxide catalyst according to an embodiment of the present invention includes at least one selected from the group consisting of transition metals (atomic number 21 to 29, 39 47, 72 to 79, or 104 to 108), lanthanide (atomic number 57 to 71), post-transition metals (atomic number 13, 30 to 31, 48 to 50, 80 to 84, and 112), and metalloids (atomic number 14, 32 to 33, 51 to 52, and 85) in the periodic table, or a combination thereof, as an active site.

[0045] A method of preparing a metal oxide catalyst according to an embodiment of the present invention uses supercritical CO.sub.2 extraction to remove a synthetic solvent used to dissolve an active site precursor, wherein the stoichiometry of a metal and oxygen is controlled by adjusting the firing (or calcination) condition. In addition, a metal-oxygen coordination bond or the like may be controlled by implementing various metal oxide structures, and thereby the redox properties and the distribution/number/intensity of Brönsted acid sites, Lewis acid sites, surface labile oxygen species, oxygen vacancies, etc. present on the surface of a metal oxide may be preferably controlled irrespective of the type of metal used for preparing the active site.

[0046] For example, in the case of manganese oxide, 1) a synthetic solvent used to dissolve a manganese oxide precursor is removed by using supercritical CO.sub.2 extraction, wherein the firing (or calcination) conditions may be adjusted, thereby 2) diversifying the structure to α-MnO2, γ-MnO2, Mn2O3, Mn3O4, etc., and 3) controlling the distribution and manganese oxidation number on a support surface of each structure and 4) the coordination number of Mn—O bonds inherent in the above structures, the number/intensity of vacancies or defects and the redox properties may be adjusted, thereby 5) controlling selective activation of the N—O bonds and N—H bonds and related performance.

[0047] The metal oxide active site according to an embodiment of the present invention may have porosity, and may be dispersed in a porous support described below.

[0048] The metal oxide active site according to an embodiment of the present invention may have a diameter (maximum diameter) of 0.1 nm to 500 μm, and may have a composition range of 10.sup.−4 to 50 parts by weight based on 100 parts by weight of the support.

[0049] The metal oxide catalyst according to an embodiment of the present invention includes at least one selected from the group consisting of alkaline earth metals (atomic number 4, 12, 20, 38, 56, and 88), transition metals (atomic number 21-29, 39-47, and 72-79, or 104-108), lanthanide (atomic number 57-71), post-transition metals (atomic number 13, 30-31, 48-50, 80-84, and 112), and metalloids (atomic number 14, 32-33, 51-52, and 85) in the periodic table, or carbon (C), or a combination thereof, as the support.

[0050] The support uses supercritical CO.sub.2 extraction to 1) minimize the structural collapse and the damage to porosity and, 2) improve the dispersity of active sites in the pores or the support surface, and 3) preferably control the redox properties and the distribution/number/intensity of surface labile oxygen species and oxygen vacancies irrespectively of the type of metal used for preparing the support.

[0051] Specifically, the effect of the supercritical CO.sub.2 extraction proposed in the present invention is enormous when applied to a support having microporosity. This is because the supercritical carbon dioxide fluid significantly weakens the interaction between micropores and the synthetic solvent that dissolves the active site precursor. That is, since the surface tension and capillary effect of the synthetic solvent, which are problematic in the process of removing the synthetic solvent, can be considerably weakened, the collapse of micropores can be reduced when the synthetic solvent is removed, and the microporosity of the support may be maintained even after the calcination treatment. Therefore, the supercritical CO.sub.2 extraction method may ultimately implement the surface properties of active sites desirable for selective activation of bonds inherent in reactants, such as N—O bonds, N—H bonds, C—O bonds, or O—H bonds, within a range that does not inhibit the dispersity of the active sites dispersed in the micropores.

[0052] Specifically, the effect of the supercritical CO.sub.2 extraction proposed in the present invention is enormous when applied to a reducible support (e.g., CeO.sub.2 or TiO.sub.2) that may contain labile oxygen species or oxygen vacancies on the surface thereof. This is because 1) the redox properties and the number/distribution of labile oxygen species or oxygen vacancies exposed to the surface after calcination treatment can be maximized by efficiently removing organic matter/impurities contained in the synthetic solvent or active site precursor before the calcination treatment and 2) the redox properties and the interaction (bond strength) between the labile oxygen species or oxygen vacancies and the catalytic reactant can be controlled by controlling the supercritical CO.sub.2 extraction or calcination treatment conditions.

[0053] Hereinafter, the present invention will be described in detail by explaining embodiments of the invention. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein.

Embodiments 1 and 2: Preparation of Mn and Mn (Sc CO2) Catalysts

[0054] 37.5 mL of distilled water in which 6.9 g of sulfuric acid (98% H.sub.2SO.sub.4) was dissolved was heated to 50° C., and then added with 11.25 g of TiOSO.sub.4, a titanium salt, and dissolved for 30 minutes. Thereafter, after adding thereto 75 g of urea (CO(NH.sub.2).sub.2) and 500 mL of distilled water, the temperature of the mixture was raised to 100° C. and the mixture was stirred for 18 hours. A formed intermediate product was cooled to 25° C. and then filtered/washed with distilled water, and an obtained solid was exposed for about 30 minutes in a supercritical CO2 (99.99%) fluid (Sc CO.sub.2) obtained under the conditions of a temperature of 60 to 70° C. and a pressure of 90 to 100 atm and thereafter subjected to calcination at 400° C. for 3 hours to obtain a titanium oxide (TiO.sub.2) with hierarchical porosity having both mesoporosity and microporosity. The catalysts of Embodiments 1 and 2 were synthesized using TiO.sub.2 as a support. In order to synthesize the catalysts of Embodiment 1, 1.95 g of Mn(NO.sub.3).sub.2.XH.sub.2O, a manganese salt, and 3.4 g of TiO.sub.2 were added to 250 mL of distilled water and then stirred at 25° C. for 30 minutes, and then the pH of the liquid mixture was adjusted to 10 by using NH.sub.4OH. After stiffing at 25° C. for 18 hours, the mixture was dehydrated and subjected to calcination at 400° C. for 3 hours to obtain the catalyst of Embodiment 1, which was referred to as Mn. To synthesize the catalysts of Embodiment 2, 1.95 g of Mn(NO.sub.3).sub.2.XH.sub.2O, a manganese salt, and 3.4 g of TiO.sub.2 were added to 250 mL of distilled water, and stiffed at 25° C. for 30 minutes, and then the pH of the liquid mixture was adjusted to 10 by using NH.sub.4OH. After stiffing at 25° C. for 18 hours, the mixture was filtered/washed with distilled water. An obtained solid was exposed for about 30 minutes in a supercritical CO.sub.2 (99.99%) fluid (Sc CO.sub.2) obtained under the conditions of a temperature of 60 to 70° C. and a pressure of 90 to 100 atm and then subjected to calcination at 400° C. for 3 hours to obtain the catalyst of Embodiment 2, which was referred to as Mn(Sc CO.sub.2).

Experimental Example 1: Analysis of Catalytic Characters

[0055] Surface morphologies of the catalysts synthesized according to Embodiments 1 and 2 were analyzed using scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM), and the results thereof are shown in FIG. 1A to 1D. Referring to FIG. 1A to 1D, they are shown that TiO.sub.2 agglomerates having grain sizes (maximum diameters) of several hundred nanometers to several hundred micrometers configure porous supports in the synthesized catalysts.

[0056] In order to check porosity of the catalysts of Embodiments 1 and 2, a nitrogen gas (N.sub.2) physisorption test was performed to measure the micropore surface areas (SMICRO) and the mesopore surface areas (SMESO) of the catalysts by applying the non-localized density functional theory. In addition, components of the catalysts synthesized according to Embodiments 1 and 2 were analyzed using X-ray fluorescence (XRF). The results thereof are shown in Table 2.

TABLE-US-00002 TABLE 2 Catalyst content (mmol g−1) Theoretical Observed Catalyst S.sub.MICRO S.sub.MESO value value Embodiment 1 25.5 m.sup.2 g.sup.−1 100.4 m.sup.2 g.sup.−1 Mn: 2.73 Mn: 2.75 Embodiment 2 21.9 m.sup.2 g.sup.−1  .sup. 93.6 m.sup.2 g.sup.−11 Mn: 2.73 Mn: 2.74

[0057] The results of measuring the micropore surface areas (SMICRO) and mesopore surface areas (SMESO) showed that the catalysts synthesized according to Embodiment 1 and 2 had hierarchical porosity in which micropores and mesopores were mixed. In addition, it was confirmed that there was reasonable agreement between the theoretical and observed values for the content of catalytic active sites. Specifically, in Embodiments 1 and 2, it can be seen that the catalysts had approximately 15 wt % of Mn (˜2.73 mmol Mn g.sup.−1), which indicates that the catalysts synthesized by the supercritical CO.sub.2 extraction and the catalysts synthesized without the supercritical CO.sub.2 extraction had similar contents of active sites.

[0058] Crystal structures of Embodiments 1 and 2 were analyzed using an X-ray diffractometer, and X-ray diffraction (XRD) patterns obtained as results thereof are shown in FIG. 2. Referring to FIG. 2, it can be seen that all of the catalysts in Embodiments 1 and 2 included crystal planes of anatase phase having tetragonal crystal structure which indicates a TiO.sub.2 support. Meanwhile, crystal planes of α-MnO.sub.2 phase having tetragonal crystal structure, γ-MnO.sub.2 phase having orthorhombic crystal structure, α-Mn.sub.2O.sub.3 phase having cubic crystal structure, and Mn.sub.3O.sub.4 phase having tetragonal crystal structure were observed on the X-ray diffraction pattern of Embodiment 1, but the crystal planes of manganese oxides described above were not observed on the X-ray diffraction pattern of Embodiment 2. This may be because the bulk crystal structure of the manganese oxides of Embodiment 2 synthesized by the supercritical CO.sub.2 extraction process was small to be detected through X-ray diffraction analysis.

[0059] Accordingly, the catalysts of Embodiments 1 and 2 were analyzed using a selected area electron diffraction (SAED) pattern, and the results thereof are shown in FIGS. 3A and 3B. Referring to FIGS. 3A and 3B, as in the result of X-ray diffraction analysis, (1 0 1) and (0 0 4) planes of anatase phase having tetragonal crystal structure were commonly observed (see red concentric circles). In addition, crystal planes of α-MnO.sub.2 phase having tetragonal crystal structure (yellow concentric circles 1 and 3), γ-MnO.sub.2 phase having orthorhombic crystal structure (yellow concentric circles 2), α-Mn.sub.2O.sub.3 phase having cubic crystal structure (yellow concentric circles 6), and Mn.sub.3O.sub.4 phase having tetragonal crystal structure (yellow concentric circles 1-6) were also commonly observed. Thus, it was confirmed that the manganese oxides, which were the catalytic active sites of Embodiments 1 and 2, were successfully dispersed in the TiO.sub.2 support having hierarchical porosity.

[0060] In order to check interaction between the active site (manganese oxide) and the support (TiO.sub.2) of the catalysts of Embodiments 1 to 2, the H.sub.2-temperature programmed reduction (H.sub.2-TPR) technique was used. Results (H.sub.2-TPR spectra) thereof are shown in FIGS. 4A and 4B. Referring to FIGS. 4A and 4B, five bands were observed in the H.sub.2-TPR spectra of Embodiments 1 and 2, they respectively represent the degree of interaction between manganese oxide and TiO.sub.2 (red band), reduction of Mn.sup.4+ contained in manganese oxide to Mn.sup.3+ (green band), reduction of Mn.sup.3+ in manganese oxide to Mn.sup.2+/Mn.sup.3+ (blue band), reduction of Mn.sup.2+/Mn.sup.3+ in manganese oxide to Mn.sup.2+ (light blue band), reduction of Ti.sup.4+ in TiO.sub.2 to Ti.sup.3+ (purple band). More interactions (red band) between manganese oxide and TiO.sub.2 were observed in Embodiment 2 than Embodiment 1 (7.4% in Embodiment 1 and 20.6% in Embodiment 2). This indicates that, as compared to Embodiment 1, Embodiment 2 reduced the chance for interaction between catalyst poisoning species (for example, SO.sub.2 in the case of SCR reaction) contained in an exhaust gas and manganese oxide, which is an active site, during the reaction, thereby minimizing a poisoning phenomenon caused by the poisoning species, and the resistance of the catalyst to the poisoning species can be increased.

[0061] In order to analyze the redox properties of the catalysts of Embodiments 1 and 2, the X-ray photoelectron(XP) spectroscopy was used in the O 1s region, and the results thereof are shown in FIGS. 5A and 5B. Referring to FIGS. 5A and 5B, in the catalysts of Embodiments 1 and 2, oxygen species (O.sub.α′) existing in H.sub.2O chemically adsorbed on the catalyst surface, labile oxygen species (O.sub.α), and oxygen species (O.sub.β) existing in a catalyst lattice were observed. In the case of Embodiment 2, it was observed that a greater amount of labile oxygen species (O.sub.α) was contained on the surface than that of Embodiment 1. This indicates that the catalyst of Embodiment 2 supplied a greater amount of labile oxygen species during the catalytic reaction, compared to the catalyst of Embodiment 1, thereby improving the rate and performance (conversion and selectivity) of the catalytic reaction.

[0062] Hereinafter, with reference to FIGS. 6 to 11, results of performance analysis in selective catalytic reduction (SCR) and selective catalytic oxidation (SCO) reactions of the catalysts according to Embodiments 1 and 2 of the present invention will be described.

Experimental Example 2: Performance Analysis of SCR Reaction (1)

[0063] The performance of SCR process was measured using the catalysts of Embodiments 1 and 2. FIGS. 6A and 6B show a NO.sub.X conversion X.sub.NOX and nitrogen (N.sub.2) selectivity S.sub.N2 in a temperature range of 150° C. to 400° C. The SCR process was performed under the conditions that a reaction fluid contained 200 ppm of NO.sub.X, 200 ppm of NH.sub.3, 3 vol % of O.sub.2, 6 vol % of H.sub.2O, 500 ppm of SO.sub.2, and an inert gas of N.sub.2, a total flow rate was 500 mLmin.sup.−1, and a space velocity was 30,000 hr.sup.−1. Referring to FIGS. 6A and 6B, it can be seen that the catalyst of Embodiment 2 exhibited improved performance in the temperature range of 150° C. to 400° C. compared to that of Embodiment 1, which indicates that the catalyst of Embodiment 2 synthesized through the supercritical CO.sub.2 extraction exhibited improved N.sub.2 productivity due to the SCR reaction in a low temperature range (200° C. or less), lower side reactant (N.sub.2O) productivity due to the SCR reaction in a middle temperature range (200° C. to 280° C.), and improved N.sub.2 productivity due to the SCR and SCO reactions in a high temperature range (over 280° C.), when compared to the catalyst of Embodiment 1 synthesized by the conventional method. In addition, this indicates that the catalyst of Embodiment 2 has Brönsted acid, Lewis acid, and redox properties which are more desirable to selectively activate N—O bonds or N—H bonds than those of the catalyst of Embodiment 1 in the surface thereof.

Experimental Example 3: Performance Analysis of SCR Reaction (2)

[0064] The performance of SCR process was measured using the catalysts of Embodiments 1 and 2. A decrease trend of a NO.sub.X conversion X.sub.NOX in the presence or absence of O2 at 180° C. (1 to 4 hours) was divided by the initial NO.sub.X conversion X.sub.NOX,0 and the result (X.sub.NOX/X.sub.NOX,0) is shown in FIG. 7. The SCR process was performed under the conditions that a reaction fluid contained 200 ppm of NO.sub.X, 200 ppm of NH.sub.3, 3 vol. % of O2, 6 vol. % of H.sub.2O, and an inert gas of N.sub.2, a total flow rate was 500 mLmin.sup.−1, and a space velocity was 30,000 hr.sup.−1. Referring to FIG. 7, it can be seen that the catalyst of Embodiment 2 exhibited more desirable redox properties to selectively activate N—O bonds or N—H bonds in the low temperature range (180° C.), when compared to that of Embodiment 1. This is manifested by the fact that the catalyst of Embodiment 2 synthesized through the supercritical CO.sub.2 extraction had a higher value of X.sub.NOX/X.sub.NOX,0 and a lower rate of decrease of the value of X.sub.NOX/X.sub.NOX,0 in the absence of O.sub.2 compared to the catalyst of Embodiment 1.

[0065] Experimental Example 4: Performance analysis of SCO reaction (1) The performance of SCO process was measured using the catalysts of Embodiments 1 and 2. FIGS. 8A and 8B show an ammonia (NH.sub.3) conversion X.sub.NH3 and selectivity S.sub.N2/S.sub.NOX/S.sub.N2O to products (N.sub.2, NO.sub.X, and N.sub.2O) in a temperature range of 150° C. to 400° C. The SCO process was performed under the conditions that a reaction fluid contained 200 ppm of NH.sub.3, 3 vol. % of O2, 6 vol. % of H.sub.2O, and an inert gas of N.sub.2, a total flow rate was 500 mLmin.sup.−1, and a space velocity was 30,000 hr.sup.−1. Referring to FIGS. 8A and 8B, it can be seen that the catalyst of Embodiment 2 exhibited improved performance in the temperature range of 150° C. to 400° C. compared to that of Embodiment 1, which is manifested by the fact that the catalyst of Embodiment 2 synthesized through the supercritical CO.sub.2 extraction had improved values of X.sub.NOX due to the SCO reaction, a higher selectivity S.sub.N2 to a desirable product, and lower selectivity S.sub.NOX and S.sub.N2O to undesirable products at the same reaction temperature compared to the catalyst of Embodiment 1 synthesized by the conventional method. This indicates that the catalyst of Embodiment 2 has Brönsted acid, Lewis acid, and redox properties that are more preferable to selectively activate N—H bonds than those of the catalyst of Embodiment 1 in the surface thereof.

Experimental Example 5: Performance Analysis of SCO Reaction (2)

[0066] The performance of SCO process was measured using the catalysts of Embodiments 1 and 2. A decrease trend of a NH.sub.3 conversion X.sub.NH3 in the presence or absence of O.sub.2 at 350° C. (1 to 4 hours) was divided by the initial NH.sub.3 conversion X.sub.NH3,0 and the result (X.sub.NH3/X.sub.NH3,0) thereof is shown in FIG. 9A. Also, the selectivity S.sub.N2 to nitrogen, which is a desirable product, shown in FIG. 9B. The SCO process was performed under the conditions that a reaction fluid contained 200 ppm of NH.sub.3, 3 vol. % of O2, 6 vol. % of H.sub.2O, and an inert gas of N.sub.2, a total flow rate was 500 mLmin.sup.−1, and a space velocity was 30,000 hr.sup.−1. Referring to FIGS. 9A and 9B, it can be seen that the catalyst of Embodiment 2 exhibited more desirable redox properties to selectively activate N—H bonds in the high temperature range (350° C.), when compared to that of Embodiment 1. This is manifested by the fact that the catalyst of Embodiment 2 synthesized through the supercritical CO.sub.2 extraction had a higher value of X.sub.NH3/X.sub.NH3,0, a lower rate of decrease of the value of X.sub.NH3/X.sub.NH3,0, and a higher value of S.sub.N2 in the absence of O.sub.2 compared to the catalyst of Embodiment 1.

Experimental Example 6: Performance Analysis of SCR Reaction (3)

[0067] The performance of SCR process for the catalysts of Embodiments 1 and 2 was measured in a reaction fluid contained 200 ppm of NO.sub.X, 200 ppm of NH3, 3 vol % of O.sub.2, 6 vol % of H.sub.2O, and an inert gas of N.sub.2, at 180° C. and at a space velocity of 30,000 hr.sup.−1, and the results thereof are shown in FIG. 10. Specifically, based on the poisoning phenomenon of the catalyst surface caused by H.sub.2O/SO.sub.2/AS (ammonium sulfate)/ABS (ammonium bisulfate), etc., a decrease trend of the NO.sub.X conversions X.sub.NOX of the catalysts was observed. Referring to FIG. 10, the catalyst of Embodiment 2 exhibits a higher NO.sub.X conversion X.sub.NOX in the presence and absence of SO.sub.2 in the low temperature range (180° C.) compared to that of Embodiment 1. This indicates that the catalyst of Embodiment 2 has stronger resistance to poisons supplied/generated during the reaction compared to that of Embodiment 1. This indicates that the catalyst of Embodiment 2 synthesized through the supercritical CO.sub.2 extraction imparts excellent resistance to poisons (H.sub.2O/SO.sub.2/AS/ABS) compared to the catalyst of Embodiment 1 synthesized by the conventional method, and has an improved life span.

Experimental Example 7: Performance Analysis of SCR Reaction (4)

[0068] The performance of SCR process for the catalysts of Embodiments 1 and 2 was measured in a reaction fluid contained 200 ppm of NO.sub.X, 200 ppm of NH.sub.3, 3 vol % of O.sub.2, 6 vol % of H.sub.2O, and an inert gas of N.sub.2, at 180° C. and 200° C. and at a space velocity of 30,000 hr.sup.−1, and the results thereof are shown in FIG. 11. Specifically, NO.sub.X conversions X.sub.NOX of the catalysts were divided by the corresponding initial NO.sub.X conversions X.sub.NOX,0 (in the absence of SO.sub.2). In addition, based on the poisoning phenomenon of the catalyst surface caused by H.sub.2O/SO.sub.2/AS (ammonium sulfate)/ABS (ammonium bisulfate), the time required for the catalysts to show 65% of performance (X.sub.NOX/X.sub.NOX,0˜0.65) compared to the initial performance was measured. Referring to FIG. 11, it was observed that the catalyst of Embodiment 2 (10 hours at 180° C. and 18 hours at 200° C.) exhibited improved resistance to poisons in a low temperature range compared to the catalyst of Embodiment 1 (6 hours at 180° C. and 15 hours at 200° C.). This indicates that the catalyst of Embodiment 2 synthesized through the supercritical CO.sub.2 extraction imparts excellent resistance to poisons (H.sub.2O/SO.sub.2/AS/ABS) compared to the catalyst of Embodiment 1 synthesized by the conventional method, and has an improved life span.

[0069] According to one aspect of the present invention made as described above, a catalyst in which oxides of one or more metals selected from the above-described periodic table are dispersed in a support is synthesized using supercritical CO.sub.2 extraction, so that the distribution/number/intensity of Brönsted acid sites, Lewis acid sites, surface labile oxygen species, oxygen vacancies, etc., present on a surface of the catalyst and the redox properties can be preferably controlled.

[0070] In addition, the metal oxide catalyst prepared using the supercritical CO.sub.2 extraction enables selective activation of bonds inherent in reactants, for example, N—O bond, N—H bond, C—O bond, and O—H bond, so that it is possible to implement a high rate and an increased conversion or selectivity compared to catalysts synthesized by previously reported methods (filtration, washing, or thermal drying). Catalysts synthesized based on the advantages provided by the above-described supercritical CO.sub.2 extraction may have remarkably improved reactivity and durability compared to catalysts synthesized by conventional methods.

[0071] However, the above-described effects are merely examples and the scope of the present invention is not limited thereto.

[0072] While the present invention has been particularly shown and described with reference to embodiments thereof, it will be understood by one of ordinary skill in the art that various changes in form and details may be made therein without departing from the scope of the present invention as defined by the following claims.