Oxidation catalyst, method for preparing same, and filter for exhaust gas purification comprising same

Abstract

The present invention relates to an oxidation catalyst, a method for preparing the same, and a filter for exhaust gas purification comprising the same and, more specifically, to an oxidation catalyst, a method for preparing the same, and a filter for exhaust gas purification comprising the same, the oxidation catalyst being formed by comprising an amorphous metal alloy powder, thereby being preparable at a low cost, being capable of enhancing purification efficiency for exhaust gas when applied to the filter for exhaust gas purification, and being capable of deriving reliability enhancement for operation of an exhaust gas purifier having the filter for exhaust gas purification mounted therein. To this end, the present invention provides an oxidation catalyst, a method for preparing the same, and a filter for exhaust gas purification comprising the same, the oxidation catalyst characterized by being coated onto the carrier surface of the filter for exhaust gas purification and being formed by comprising an amorphous metal alloy powder.

Claims

1. A method of preparing an oxidation catalyst that coats a surface of a carrier of an exhaust gas purification filter, the method comprising: a melting step of melting a metal and a master alloy, producing a molten metal alloy comprising the metal and the master alloy; a rapid cooling step of producing an amorphous metal alloy by rapidly cooling the molten metal alloy; and a powdering step of converting the amorphous metal alloy into powder, wherein the powdering comprises pulverization after vacuum atomization or melt spinning.

2. The method of claim 1, wherein, in the melting step, at least one element selected from the group consisting of Fe, Ni, Mn, Co, Zr, and Pt and at least two elements selected from the group consisting of B, Y, Ti, P, Pd, Be, Si, C, Ag, Na, Mg, Ga, and Al are used as the metal and the master alloy.

3. The method of claim 2, wherein, in the melting step, Fe, B, Y, Ti, and Pt are used as the metal and the master alloy.

4. The method of claim 3, wherein, in the melting step, Fe, B, Y, Ti, and Pt are used as the metal and the master alloy at ratios of at least 50 atomic % of Fe, 10 to 30 atomic % of B, 5 to 20 atomic % of Y, and 0 to 10 atomic % of Ti+Pt.

5. The method of claim 1, wherein, in the rapid cooling step, the molten metal alloy is cooled at a cooling rate ranging from 100 C./s to 1,000,000 C./s.

6. The method of claim 1, further comprising a step of increasing a surface roughness value of the amorphous metal alloy after the powdering step.

7. The method of claim 1, further comprising an oxidation step of oxidizing the amorphous metal alloy powder at a temperature ranging from 300 C. to 600 C. in an oxygen atmosphere.

8. The method of claim 7, wherein, after the oxidation step, the oxidation catalyst comprising the amorphous metal alloy powder has a performance of converting CO into CO.sub.2 of 95% or higher at 150 C. and does not react with NO.

9. The method of claim 7, wherein, after the oxidation step, the oxidation catalyst comprising the amorphous metal alloy powder has an oxidation performance for NH.sub.3 of 75% or higher at 300 C. and produces no NO.sub.2 by-product during oxidation of NH.sub.3.

10. The method of claim 7, wherein, in the oxidation step, a surface structure of the amorphous metal alloy changes from an FeO structure, in which a degree of oxidation of Fe in the amorphous metal alloy is +2, to an Fe.sub.2O.sub.3 structure, in which a degree of oxidation of Fe in the amorphous metal alloy is +3, as a heat treatment temperature increases.

Description

DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a configuration view schematically illustrating a typical catalytic converter;

(2) FIG. 2 is a conceptual view illustrating the atomic structure of a crystalline metal;

(3) FIG. 3 is a conceptual view illustrating the atomic structure of an amorphous metal;

(4) FIG. 4 is a flowchart sequentially illustrating the process steps of a method of preparing an oxidation catalyst according to an exemplary embodiment;

(5) FIG. 5 is SEM micrographs illustrating the surface shapes of oxidation catalysts prepared by the method of preparing an oxidation catalyst according to the exemplary embodiment;

(6) FIG. 6 is an XRD graph illustrating oxidation catalysts prepared by the method of preparing an oxidation catalyst according to the exemplary embodiment;

(7) FIG. 7 is a graph illustrating the results of CO oxidation tests performed on oxidation catalysts prepared by the method of preparing an oxidation catalyst according to the exemplary embodiment;

(8) FIG. 8 is a graph illustrating the results of CO oxidation tests performed, after pretreatment, on oxidation catalysts prepared by the method of preparing an oxidation catalyst according to the exemplary embodiment;

(9) FIG. 9 is a graph illustrating an NO oxidation test result performed on the oxidation catalyst prepared by the method of preparing an oxidation catalyst according to the exemplary embodiment;

(10) FIG. 10 is a graph illustrating NO-TPD test results performed on the oxidation catalyst prepared by the method of preparing an oxidation catalyst according to the exemplary embodiment;

(11) FIG. 11 is a graph illustrating NH.sub.3 oxidation test results performed, after pretreatment, on the oxidation catalyst prepared by the method of preparing an oxidation catalyst according to the exemplary embodiment;

(12) FIG. 12 is a graph illustrating NO.sub.2 concentration measurements during the NH.sub.3 oxidation test performed, after pretreatment, on the oxidation catalyst prepared by the method of preparing an oxidation catalyst according to the exemplary embodiment;

(13) FIG. 13 is a graph illustrating XPS changes depending on oxidation performed on the oxidation catalyst prepared by the method of preparing an oxidation catalyst according to the exemplary embodiment;

(14) FIG. 14 is TEM micrographs of the oxidation catalyst prepared by the method of preparing an oxidation catalyst according to the exemplary embodiment; and

(15) FIG. 15 is an XRD graph illustrating the oxidation catalyst prepared by the method of preparing an oxidation catalyst according to the exemplary embodiment before and after oxidation.

MODE FOR INVENTION

(16) Hereinafter, reference will be made in detail to an oxidation catalyst, a method of preparing the same, and an exhaust gas purification filter including the same according to the present disclosure, in conjunction with the accompanying drawings, in which exemplary embodiments thereof are illustrated.

(17) In addition, in the description of the present invention, detailed descriptions of known functions and components will be omitted in the case that the subject matter of the present invention is rendered unclear by the inclusion thereof.

(18) An oxidation catalyst according to an exemplary embodiment is a catalyst coating the surface of a carrier of an exhaust gas purification filter disposed in an exhaust gas purifier provided in a power plant, an incinerator, a vessel, or the like to play a part in or promote a chemical reaction for converting harmful components, such as Co or NH.sub.3, contained in exhaust gases, into non-harmful components. The oxidation catalyst according to the present embodiment contains an amorphous metal alloy powder.

(19) Comparing an amorphous metal with a crystalline metal with reference to FIG. 2 and FIG. 3, the amorphous metal is characterized by having very high surface energy and activity, since the atomic structure of the surface thereof is highly disordered and a plurality of dangling bonds representing defects between atomic bonds are formed. In addition, the amorphous metal may have higher erosion resistance and higher mechanical strength than the crystalline metal, due to physical, chemical, and structural factors.

(20) Thus, the oxidation catalyst according to the present embodiment is used as a catalyst for purifying exhaust gases on the basis of such characteristics of the amorphous metal. When the oxidation catalyst according to the present embodiment formed from an amorphous metal alloy powder is used to purify exhaust gases, it is possible to improve the efficiency of exhaust gas purification compared to prior-art processes in which noble metal catalysts are used. In addition, the oxidation catalyst can be prepared at low cost, such that an exhaust gas purification filter having the oxidation catalyst provided as an exhaust gas purification catalyst can be fabricated at a significantly low cost.

(21) In addition, the amorphous metal alloy has superior durability, since the amorphous metal alloy is neither condensed nor crystallized by exhaust gases having a temperature ranging from 500 C. to 600 C. Thus, the oxidation catalyst formed from the amorphous metal alloy is not shed from the carrier of the exhaust gas purification filter when exposed to exhaust gases over a long period of time, thereby contributing to improvements in the reliability of the operation of an exhaust gas purifier in which the exhaust gas purification filter including the oxidation catalyst is disposed.

(22) The oxidation catalyst as described above may be formed from an amorphous metal alloy powder produced by mixing at least one selected from the group consisting of Pt, Ni, Fe, Co, and Zr and at least one selected from the group consisting of B, P, Pd, Be, Si, C, Ag, Na, Mg, Ga, Y, Ti, and Al. That is, the composition of the oxidation catalyst according to the present embodiment may include three or more elements.

(23) In addition, the particle size of the amorphous metal alloy powder of the oxidation catalyst may range from 0.1 m to 10 m. Furthermore, it is preferable that the surface roughness of the oxidation catalyst of the oxidation catalyst ranges from 1 nm to 10 nm such that the oxidation catalyst has an optimal specific surface area for a catalyst.

(24) Hereinafter, reference will be made to a method of preparing an oxidation catalyst according to an exemplary embodiment.

(25) As illustrated in FIG. 4, the method of preparing an oxidation catalyst according to the present embodiment is a method of preparing an oxidation catalyst that coats the surface of a carrier of an exhaust gas purification filter disposed in an exhaust gas purifier provided in a power plant, an incinerator, a vessel, or the like, and includes a melting step S1, a rapid cooling step S2, and powdering step S3.

(26) First, the melting step S1 is a step of melting a metal and a master alloy. That is, in the melting step S1, a molten liquid metal alloy is prepared by inserting the metal and the master alloy into a crucible and then heating the metal and the master alloy. In the melting step S1, at least one element selected from the group consisting of Fe, Ni, Mn, Co, Zr, and Pt and at least two elements selected from the group consisting of B, Y, Ti, P, Pd, Be, Si, C, Ag, Na, Mg, Ga, and Al may be used as the metal and the master alloy. For example, in the melting step S1, Fe, B, Y, Ti, and Pt may be selected as the metal and the master alloy. In this case, in the melting step S1, the content ratios of the metal and the master alloy may be controlled to be at least 50 atomic % of Fe, 10 to 30 atomic % of B, 5 to 20 atomic % of Y, and 0 to 10 atomic % of Ti+Pt.

(27) The subsequent rapid cooling step S2 is a step of rapidly cooling the molten metal alloy. That is, the rapid cooling step S2 produces an amorphous metal alloy by rapidly cooling the molten metal alloy. In this regard, in the rapid cooling step S2, the molten metal alloy can be cooled at a cooling rate ranging from 100 C./s to 1,000,000 C./s. When the molten metal alloy is rapidly cooled as described above, the molten metal alloy solidifies with a disordered atomic arrangement like that of glass, thereby forming the amorphous metal alloy.

(28) The final powdering step S3 is a step of converting the amorphous metal alloy into powder. The powdering step S3 may be vacuum atomization or melt spinning. That is, the powdering step S3 may convert the amorphous metal alloy into a coarse powder, the particle sizes of which range from 10 m to 50 m, through vacuum atomization, and then convert the coarse powder into a fine powder, the particle sizes of which range from 0.1 m to 10 m, through additional mechanical milling. In addition, the powdering step S3 may convert the amorphous metal alloy into an amorphous metal ribbon through melt spinning and then convert the amorphous metal ribbon into powder through mechanical milling.

(29) When the powdering step S3 as described above is completed, an oxidation catalyst formed from the amorphous metal alloy powder is prepared.

(30) The method of preparing an oxidation catalyst according to the present embodiment may further include a step of increasing the surface roughness of the amorphous metal alloy powder after the powdering step S3. Here, the surface roughness of the amorphous metal alloy powder is increased in order to improve the performance of the oxidation catalyst through obtaining a greater specific surface area and to increase the compatibility and bonding force of the exhaust gas purification filter to a ceramic carrier through obtaining the rougher surfaces. This step may be a process of forming nanoscale structures on the surface of the amorphous metal alloy powder through mechanical pulverization technology using fluid. Through this process, the metal alloy powder having an optimal specific surface area, the level of surface roughness of which ranges from 1 nm to 10 nm, may be prepared.

(31) Hereinafter, reference will be made to the results of tests performed on the characteristics of oxidation catalysts prepared by the method of preparing an oxidation catalyst according to the present embodiment in conjunction with FIG. 5 to FIG. 15.

(32) FIG. 5 is scanning electron microscopy (SEM) micrographs illustrating the surface shapes of oxidation catalysts prepared by the method of preparing an oxidation catalyst according to the present embodiment. In the present embodiment, oxidation catalyst samples formed from (Fe.sub.72B.sub.22Y.sub.6)Ti.sub.2 and ((Fe.sub.72B.sub.22Y.sub.6)Ti.sub.2)Pt.sub.2 were prepared. Both of the two samples were prepared to have reproducibility through a process of repeated experimentation.

(33) Master alloys, the compositions of which include predetermined ratios of the above-described elements, were uniformly prepared using an arc melter. Amorphous ribbons manufactured using a melt spinner were converted into powder having the surface shapes and particle sizes as illustrated in FIG. 5 through Spex milling or ball milling pulverization. Through the analysis of the SEM micrographs regarding the surface shapes and average particle sizes, it was appreciated that the particle sizes of the manufactured powders range from 5 m to 10 m.

(34) FIG. 6 illustrates the results of XRD analysis intended to examine the atomic structures of oxidation catalysts formed from amorphous metal alloy powders prepared according to the present embodiment. It can be appreciated that amorphous metal ribbons were manufactured by the above-described method as expected, since the ribbons manufactured had no XRD peaks but had wide XRD patterns representing the uniform amorphous structure. Since the powder samples had the same patterns after Spex milling, it can be appreciated that no crystallization occurred during the milling.

(35) FIG. 7 is a graph illustrating CO oxidation performance measured to examine the performance of oxidation catalysts formed from amorphous metal alloys prepared according to the present embodiment. It can be appreciated from FIG. 7 that Fe-based amorphous metal powders clearly had CO oxidation catalytic activity as the CO conversion ratios of two sample compositions reached 70% or higher at a temperature of 250 C. or higher. In addition, it can be appreciated that the sample composition including 2 atomic % of Pt element had higher oxidation catalytic performance as expected. As represented in the graph, two times of repeated experimentation showed uniform test results, thereby making the test results reliable.

(36) In FIG. 7, CO oxidation tests were performed right after the oxidation catalysts were formed from the amorphous metal alloy powders. However, pretreatment of oxidizing or reducing the samples at several hundred degrees (300 C. to 700 C.) is generally performed in order to improve the performance of the oxidation catalysts. The pretreatment makes it possible to adjust the oxidation states of the oxidation catalysts formed from the amorphous metal alloy powders and optimize catalytic activities for materials.

(37) FIG. 8 illustrates changes in the CO oxidation performance of prepared amorphous metal alloy powder samples after pretreatment (high temperature oxidation). As represented in the graph, when CO oxidation was performed after high temperature oxidation performed on samples at 400 C., 500 C., and 600 C., all the samples had improvements in oxidation performance. In particular, in the case of oxidation at 600 C., the CO conversion ratio was higher than 95% in a relatively low temperature range of 150 C. This conversion ratio exceeds the conversion ratio of Pt, a commercially-available catalyst.

(38) FIG. 9 is a graph illustrating an NO oxidation test result intended to examine whether or not an oxidation catalyst sample formed from an amorphous metal alloy prepared as described above has an effect on NO oxidation. This test was designed to measure changes in the concentration of NO.sub.2 after NO was flown over the sample. However, as illustrated in the result of FIG. 9, NO injected in a wide temperature range was determined to be discharged without being converted into NO.sub.2 at all.

(39) Thus, NO-temperature programmed desorption (NO-TPD) tests were performed in order to determine the reason why the prepared oxidation catalyst sample does not have an effect on NO oxidation differently from superior CO oxidation performance, and the results are illustrated in FIG. 10. NO-TPD is intended to examine performance for NO molecule absorption. First, samples were saturated in NO, and then, desorption signals of NO were analyzed while the temperature was being raised, whereby amounts of NO absorbed in the sample were calculated.

(40) It can be appreciated from the results of FIG. 10 that the oxidation catalyst prepared according to the present embodiment did not absorb NO at all since substantially no amount of NO desorption was measured before and after pretreatment, while the NO oxidation catalyst developed in the prior art absorbed a significant amount of NO and thus amounts of NO desorption could be clearly measured. Since the oxidation catalyst sample absorbs substantially no amount of NO, the oxidation catalyst has selective CO oxidation property.

(41) The selective CO oxidation performance of the oxidation catalyst prepared according to the present embodiment is applicable to a variety of important industrial fields. In particular, at present, commercially available Pt catalysts are generally used in order to oxidize CO in exhaust gases from power plants or incinerators. A side effect of this process is NO.sub.2 generation occurring as a side reaction. Unlikely colorless and odorless NO, NO.sub.2 forms a noticeable yellow fume with an odor when only a 15 ppm of NO.sub.2 is contained in the air. In order to overcome this problem, an additional process, such as ethanol input, is required. In contrast, the oxidation catalyst prepared according to the present embodiment is completely selective for CO and thus does not cause a side reaction, such as NO.sub.2 generation. Thus, an additional process, such as ethanol input, is not required.

(42) FIG. 11 is a graph illustrating oxidation test results performed on ammonia (NH.sub.3) using oxidation catalyst samples prepared according to the present embodiment. It can be appreciated from the result graph that both of the samples pretreated at 400 C. and 600 C. had ammonia conversion ratios of 80% or higher in a temperature range of 300 C.

(43) FIG. 12 is a graph illustrating NO.sub.2 concentration measurements intended to determine whether or not a NO.sub.2 side reaction occurs in gases converted through ammonia oxidation tests illustrated in FIG. 11. This graph indicates that the NO.sub.2 concentration was 0 ppm, i.e. no NO.sub.2 was produced. As appreciated from the NO-TPD tests, it can be interpreted as the result of selective oxidation in which an NO molecule produced during the oxidation of ammonia is not absorbed to the surface of amorphous metal alloy powder and thus is not converted to NO.sub.2. An application of selective oxidation of ammonia is found in a De-NO.sub.x SCR system that treats residual ammonia caused by slips or treats ammonia produced as a by-product in several chemical processes. When this system uses the oxidation catalyst prepared according to the present embodiment, an oxidation catalyst system can be realized without the problem of the odor and the yellow fume of NO.sub.2.

(44) FIG. 13 illustrates XPS data about Fe in metal components contained in a sample right after being powdered and samples oxidized after being powdered in the method of preparing an oxidation catalyst according to the present embodiment. It was possible to measure the binding energy levels of elements based on the XPS data and predict the atomic structure of the surface of expected materials based on the binding energy levels, thereby determining the oxidation states of metals. As appreciated from the graphs in FIG. 13, it is possible to determine the positions and intensities of peaks in three typical iron oxides, i.e. FeO (Fe oxidation state; +2), Fe.sub.2O.sub.3 (+3), and Fe.sub.3O.sub.4 (+8/3), by measuring the XPS peaks of Fe, the major element of the prepared oxidation catalysts. As appreciated from the XPS result graphs, in the case of samples prior to pretreatment, FeO, in which the degree of oxidation of Fe is +2, has most peaks. After pretreatment at 400 C., peaks gradually increase in Fe.sub.2O.sub.3 and Fe.sub.3O.sub.4. In particular, this tendency is more prominent after pretreatment at 600 C. Comparing with the results (FIG. 8) in which the CO oxidation performance is higher as the pretreatment temperature gradually increases as in the 400 C. and 600 C. pretreatments, it can be concluded that the catalytic performance will be higher as the surface structure is closer to the surface structure of Fe.sub.2O.sub.3, in which the degree of oxidation changes from +2 to +3.

(45) In general, metal oxides, such as FeO and Fe.sub.2O.sub.3, have regular crystal structures. FIG. 14 illustrates the results of transmission electron microscopy (TEM) measurements intended to analyze the atomic structures of the surfaces of iron oxides, determined based on the XPS data, in order to examine the regular crystal structures. As expected from the XPS, a crystalline portion in the amorphous structure of the initial amorphous metal caused by surface oxidation was observed from the TEM micrographs. It can be appreciated from a fast Fourier transform (FFT) image that nano crystalline structures caused by partial surface oxidation are partially distributed although not distributed over the entire surface.

(46) Although specific portions of the surface of the oxidation catalyst prepared according to the present embodiment were crystallized due to oxidation, the oxidation catalyst generally has superior durability, since the amorphous metal alloy forming the oxidation catalyst is neither condensed nor crystallized by exhaust gases having a temperature ranging from 500 C. to 600 C. As illustrated in FIG. 15, right after the powdering step and after oxidation pretreatment at 600 C. following the powdering step, amorphous XRD patterns were measured through XRD analysis. That is, regarding the overall particle structure, it can be appreciated that the same structure was maintained without crystallization after calcination performed at 600 C. for four hours. Thus, the oxidation catalyst formed from the amorphous metal alloy powder according to the present embodiment is free from deterioration, growth, or the like and thus is not shed from the surface of a carrier of an exhaust gas purification filter, even after having been exposed to exhaust gases for a long period of time. Thus, the performance of the oxidation catalyst is superior to those of the Pt and Rd catalysts of the prior art.

(47) In addition, the oxidation catalyst prepared according to the present embodiment is applied to an exhaust gas purification filter. Specifically, the exhaust gas purification filter may include a carrier, the surface of which is coated with the oxidation catalyst prepared according to the present embodiment. The exhaust gas purification filter may be fabricated by forming slurry by mixing an oxidation catalyst formed from an amorphous metal alloy powder into a solvent and coating the surface of a porous carrier with an oxidation catalyst layer by immersing the porous carrier into the slurry.

(48) Describing in greater detail, in order to fabricate the exhaust gas purification filter, first, the slurry is formed by diluting the oxidation catalyst with an aqueous solvent, an alcoholic solvent, or a mixture thereof. In this case, it is preferable that the oxidation catalyst is added at a ratio ranging from 10 wt % to 50 wt % of the solvent.

(49) The solvent as described above may include a dispersant to improve the dispersibility of the oxidation catalyst. The dispersant may include a surfactant, such as CTAB or DTAB, in order to realize dispersibility based on steric hindrance or may include at least one salt selected from among NH.sub.4OH, NaCl, and NH.sub.4Cl in order to realize electrical dispersibility.

(50) Afterwards, the oxidation catalyst layer is formed on the surface of the carrier by immersing the porous carrier into the prepared slurry. Here, it is preferable that the thickness of the oxidation catalyst layer is controlled to range from 0.5 m to 5 m.

(51) Subsequently, the solvent is evaporated by heating the porous carrier having the oxidation catalyst layer on the surface thereof at a temperature ranging from 100 C. to 150 C. for two hours. The oxidation catalyst layer is then sintered by heating the porous carrier at a temperature ranging from 450 C. to 550 C., thereby completing the fabrication of the exhaust gas purification filter.

(52) The foregoing descriptions of specific exemplary embodiments of the present disclosure have been presented with respect to the drawings. They are not intended to be exhaustive or to limit the present disclosure to the precise forms disclosed herein, and many modifications and variations are obviously possible for a person having ordinary skill in the art in light of the above teachings.

(53) It is intended therefore that the scope of the present disclosure not be limited to the foregoing embodiments, but be defined by the Claims appended hereto and their equivalents.