KR102057119B1 - A fabrication method of catalyst and exhaust gas purifying apparatus which are metal nanoparticles supported on metal oxide support treated by hydrothermal treatment for carbon monoxide oxidation - Google Patents

Abstract

The present invention provides a catalyst for hydrothermally treating carbon monoxide oxidation comprising a support comprising a metal oxide nanoparticle and a catalyst comprising a noble metal nanoparticle supported on the support, and a method of manufacturing the same. Other catalysts in the present invention may apply hydrothermal treatment conditions that are substantially similar to the actual vehicle exhaust environment to solve the catalyst inertness problem, contributing to the design of stable automotive catalysts with improved catalyst activity.

Description

A catalyst for hydrothermally treated carbon monoxide oxidation comprising a metal oxide support and noble metal nanoparticles fixed to the support, a method for manufacturing the same, and an exhaust gas purifying apparatus comprising the same on metal oxide support treated by hydrothermal treatment for carbon monoxide oxidation}

The present invention relates to a support for metal oxide nanoparticles and a catalyst for oxidizing carbon monoxide comprising noble metal nanoparticles fixed to the support, a method of manufacturing the same, and an exhaust gas purifying apparatus including the same.

Precious metal nanoparticles (NPs) such as platinum (Pt), rhodium (Rh), and palladium (Pd) supported on oxides are typical constituents of three-way-catalyst devices in automobiles. Since rare and expensive metals are used in the devices, there have been many attempts to reduce the content of metals while maintaining high catalytic activity. By reducing the particle size of the noble metal nanoparticles, their catalytic activity can be improved and the noble metal content can be reduced, which results in high cost efficiency. However, smaller metal nanoparticles tend to thicken during catalysis, due to the sintering of the metal nanoparticles. This is due to the high surface energy of the metal nanoparticles, and this thermodynamic instability increases exponentially as the size of the metal nanoparticles decreases. Smaller nanoparticles tend to sinter more easily, so in order to ensure high catalytic activity, the nanoparticle size should be kept small and not sintered. In addition, the actual automotive exhaust environment is so severe that modern automotive catalysts must remain active at high temperatures (about 750 ° C.) and air (containing about 10% H ₂ O humidity). However, the conventional catalysts have a problem in that the surface area is reduced and the catalytically active site is lost in such an actual exhaust environment.

In the past, various studies have been conducted to prevent such deactivation, such as pretreatment, aftertreatment, or fabrication of a new catalyst structure. Among these, the redispersion phenomenon, in which the size of metal nanoparticles decreases after a certain treatment, is drawing attention as a promising strategy for regenerating sintered metal nanocatalysts. As treatments to induce redispersion, methods such as repeated oxidation-reduction, oxy-chlorination or chlorination, and methane iodide treatment have been studied.

Nagai in a method of repeated redox treatment. Y. et. According to al., the platinum nanoparticles supported on cerium oxide were redispersed under an oxidizing atmosphere. Tanabe. T. et. In al., it was confirmed that the platinum nanoparticles supported by magnesium oxide were heat-treated in air at 1073 K and reduced in the 10% hydrogen atmosphere. Wu. T. et. In al., it was confirmed that platinum nanoparticles supported on cubic cerium oxide were redispersed through repeated oxidation-reduction. Lira. E. et. In al., we reported the phenomenon that platinum nanoparticles supported on γ-Al ₂ O ₃ are oxidized and redispersed by NO. Jones. J. et. al. reported the redispersion of platinum in cerium oxide by atomic trapping. Repetitive redox treatments are difficult to apply industrially because they require rapid iteration of complex processes.

Lenormand. Oxy-chlorination or chlorination post-treatment method. F. et. In al., it was confirmed that the sintered Pt / Al ₂ O ₃ was redispersed under HCl / H ₂ O / O ₂ / N ₂ flow under 773K conditions. Galisteo. FC et. In al., the sintered Pt / Al ₂ O ₃ was reactivated by oxy-chlorination, and it was confirmed that diesel oxidation catalyst performance was restored. Lambrou. PS et. According to al., palladium nanoparticles supported on CeO ₂ -Al ₂ O ₃ were redispersed by oxy-chlorination. However, such an oxy-chlorination or chlorination post-treatment method has a complicated procedure and has difficulty in handling and treating due to the toxicity of halogen gas generated by the post-treatment.

In the process for treating methane iodide, Goguet. A. et. al. And Duan. X. P. et. According to al. methane iodide treatment increased the dispersity of gold particles to maintain catalytic activity, Sa. J. et. In al., it was confirmed that Au-I interaction after methane iodide treatment increases the dispersion of gold particles. In the case of methane iodide treatment, it is limited to gold among metals, and since gold has very low thermal stability, it is difficult to use as an actual automotive catalyst.

In order to solve the problems of the prior arts, many researchers have studied a method for improving the stability of the supported metal catalyst, and recently, cerium oxide (CeO ₂ ) as a support material for precious metal nanoparticles has been of great interest. Is dragging. This is because strong metal-supporting interactions lead to synergistic redox cycles for both the metal and cerium oxide, which is the preferred catalytic performance in a wide range of applications. Also, cerium oxide to have a high oxygen storage capacity (OSC) and redox properties by the cycle of the Ce ^{4 +} / Ce ^{3 +,} and contribute to the higher activity for CO oxidation. Among various precious metals, palladium (Pd) can be evenly dispersed on cerium oxide due to the strong interaction with cerium oxide, resulting in improved catalytic activity. Therefore, many researchers have recently shown interest in low-temperature CO oxidation with palladium supported on cerium oxide. X. Yang. et. al. reported that the redispersion effects of platinum (Pt) nanoparticles on cube-shaped cerium oxide are supported by a crossover atmosphere of oxidizing and reducing conditions. T. Wu. et. al. reported that the redispersion of cerium oxide-based supported platinum in actual vehicle exhaust environments is due to strong metal support interactions between platinum and cerium oxide. J. Lupescu. et. al. investigated the effect of treatment environment on redispersion of palladium model catalysts. B. Goldsmith. et. al. reported on the decomposition and redispersion of the tri-catalysts rhodium, palladium and platinum induced by CO and NO during oxidation of toxic gases such as NOx and CO. However, many Pd / CeO ₂ catalysts still suffer from severe deactivation during high temperature and wet air treatment, which is the actual automotive exhaust purification system environment.

In addition, when oxidizing CO using a catalyst, a carbonate layer is formed on the surface of the catalyst. In general, the various carbonates that form a layer on the surface are considered toxic and result in a decrease in CO oxidation activity. Therefore, it is very important to remove the carbonate formed on the surface in order to improve the catalytic activity and durability. In this regard, B. Schumacher. et. al. reported that inactivation of Au / TiO ₂ is caused by the accumulation and deposition of carbonate species on the surface of the catalyst and not due to the sintering of Au particles. T. Ntho. et. In al., FT-IR analysis indicated that the formation of bicarbonate on gold (Au) supported on titania nanotubes is a competitive reaction leading to catalyst deactivation. Although studies for carbonate removal are essential for efficient CO oxidation, studies to investigate this important topic have not been carried out until now, and the problem of carbonates formed during CO oxidation still persists.

Republic of Korea Patent No. 10-0206487

It is an object of the present invention to provide a catalyst comprising a metal oxide nanoparticle and a noble metal nanoparticle, a catalyst for carbon monoxide oxidation having improved catalytic activity and durability, an automobile exhaust gas purifier and a method for producing the catalyst.

The inventors pay attention to the fact that when hydrothermally treating a catalyst including metal nanoparticles immobilized on a support, the metal nanoparticles can be redispersed, and the hydrothermal process includes a support including metal oxide nanoparticles and a noble metal nanoparticle. A catalyst for the treatment of carbon monoxide oxidation has been proposed.

Accordingly, the present invention relates to a catalyst comprising a metal oxide nanoparticle and a noble metal nanoparticle, wherein the noble metal nanoparticle has a diameter of 0.3 to 1.6 nm.

The present invention also relates to an automobile exhaust gas purification apparatus comprising the catalyst for oxidizing carbon monoxide according to the present invention.

The present invention also includes forming a support including metal oxide nanoparticles, fixing a noble metal nanoparticle on the support to form a catalyst for carbon monoxide oxidation, and hydrothermally treating the catalyst for carbon monoxide oxidation. A method for producing a catalyst for oxidizing carbon monoxide.

According to the present invention there is provided a catalyst for hydrothermally treated carbon monoxide oxidation comprising a support comprising a metal oxide nanoparticle and a catalyst comprising a noble metal nanoparticle, and a hydrothermal treatment (750 ° C., 10% H ₂ O in air, 25 hours). ), The noble metal nanoparticles supported on the support including the metal oxide nanoparticles are redispersed, thereby improving the catalytic activity.

The hydrothermally treated carbon monoxide oxidation catalyst comprising a support comprising metal oxide nanoparticles and a catalyst comprising precious metal nanoparticles according to the present invention applies hydrothermal treatment conditions that are substantially similar to the actual vehicle exhaust environment to solve the catalyst deactivation problem. In addition, the catalyst activity achieves the effect of further improving, and the present invention can contribute to the design of a new automobile catalyst that is activated and stable.

The catalyst for oxidizing carbon monoxide provided according to the present invention undergoes hydrothermal treatment, and redispersion of palladium occurs and the formation of carbonate species on the surface of the catalyst is reduced. As a result, catalytic activity and durability against low temperature carbon monoxide oxidation are greatly improved.

FIG. 1 is a conceptual diagram of Pd nanoparticles being redispersed on a cerium oxide support as the hydrothermal treatment of the catalyst of the present invention and the formation of poisoning substances by the hydroxyl group, resulting in high CO oxidation activity and durability.
FIG. 2 shows (a) Pd 2 wt% Pd / CeO ₂ catalyst without heat treatment, (b) Pd 2 wt% Pd / CeO ₂ catalyst with heat treatment, (c) Pd, prepared according to Example 1 HAADF-STEM image of 2 wt% Pd / CeO ₂ catalyst. White circles represent palladium on the surface of cerium oxide. (d) is an EDS mapping image of a hydrothermally treated Pd 2 wt% Pd / CeO ₂ catalyst. Red is palladium and green is cerium.
Figure 3 (a) is a HAADF-STEM image of cerium oxide, (b) is cerium oxide, (c) 0.1% by weight of Pd untreated Pd / CeO ₂ catalyst, (d) Pd 2 weight unheated TEM image of% Pd / CeO ₂ catalyst.
4 (a) is that the HAADF-STEM and EDS mapping image of Pd 2 wt% Pd / CeO ₂ catalyst of, (b) the Pd 2 wt% Pd / CeO ₂ catalyst of the heat treatment to the heat treatment. Red is palladium and green is cerium.
5 (a) to (e) are HR-TEM images of Pd 2 wt% Pd / CeO ₂ catalyst without heat treatment. The red circle represents palladium. (f) is the palladium diameter distribution plot obtained from the HR-TEM image.
6 (a) to (e) are HR-TEM images of Pd 2 wt% Pd / CeO ₂ catalyst heat treated. The red circle represents palladium. (f) is the palladium diameter distribution plot obtained from the HR-TEM image.
7 (a) to (e) are HR-TEM images of Pd 2 / CeO ₂ catalyst of hydrothermally treated Pd 2% by weight. The red circle represents palladium. (f) is the palladium diameter distribution plot obtained from the HR-TEM image.
8 (a) Pd 2 wt% Pd / CeO ₂ catalyst without heat treatment, (b) 2 wt% Pd / CeO ₂ catalyst (T) with heat treatment Pd, (c) Pd 2 wt% Pd with hydrothermal treatment / CeO ₂ catalyst (HT), (d) 0.1% by weight of Pd / CeO ₂ catalyst without heat treatment, (e) 0.1% by weight of Pd / CeO ₂ catalyst (T) with heat treatment, (f) hydrothermally treated Pd 3d XPS spectrum of Pd 0.1 wt% Pd / CeO ₂ catalyst (HT). ^{(Pd 0: 341.5 eV (3d} 3/2) / 336 eV (3d 5/2), Pd 2 +: 342 eV (3d 3/2) /337.5 eV (3d 5/2))
FIG. 9 shows XRD results of 0.1 wt% and 2 wt% Pd / CeO ₂ catalysts before and after heat treatment or hydrothermal treatment. The PdO peak disappeared after hydrothermal treatment of 2 wt% Pd / CeO ₂ catalyst.
FIG. 10 shows (a) different moisture content, with 50 mg of Pd 2C% by weight Pd / CeO ₂ catalyst (as-made) and Pd 2C% by weight hydrotreated Pd / CeO ₂ catalyst (HT), respectively; b) CO oxidation results at different hydrothermal temperatures, (c) different hydrothermal treatment conditions and at flow rates of 100 sccm (1% CO and 1% O ₂ under He) at 120,000 ml / hrg _cat .
FIG. 11 shows the X-ray absorption near-edge structure of as-made, heat treated (T), hydrothermally treated (HT) Pd 2 wt% Pd / CeO ₂ catalyst, PdO and Pd foil ) Data.
Figure 12 (a) shows Pd K edge k ³ weighted Fourier of as-made, heat treated (T), hydrothermally treated (HT) Pd 2 wt% Pd / CeO ₂ catalyst, PdO and Pd foil It is a transformed EXAFS spectrum, and the solid line is the result of fitting experimental data and the dotted line. The spectra of PdO and Pd foils are multiplied by 1/3 and 1/4 times respectively for easy comparison. (b) shows the FT-EXAFS spectra of the as-made, heat-treated (T) and hydrothermally treated (HT) Pd 2 wt% Pd / CeO ₂ catalysts.
13 is a diagram confirming the change in the type of CO adsorption after heat treatment or hydrothermal treatment of Pd 2 wt% Pd / CeO ₂ catalyst by in-situ DRIFT measurement.
14 is a graph showing the results of oxidizing CO with catalysts prepared according to Example 1 and Comparative Example 9. FIG.
FIG. 15 shows a flow rate of 100 sccm at 120,000 ml / hrg _cat (1% CO under He) with unheat-treated, hydrothermally treated (HT) M = Pd, Pt, Rh, Ru 2 wt.% M / CeO ₂ catalyst. It is a graph showing the result of CO oxidation under 1% O ₂ ).
FIG. 16 is a flow rate of 100 sccm at 120,000 ml / hrg _cat (1% CO and 1% O under He) while controlling Pd weight ratio in an as-made, hydrothermally treated (HT) Pd / CeO ₂ catalyst. ₂ ) is a graph showing the results of oxidation of CO under.
FIG. 17 shows a flow rate of 100 sccm at 120,000 ml / hrg _cat (1% CO and 1% under He) with Pd / CeO ₂ catalyst of ₂ % by weight of (T), hydrothermally treated (HT) Pd, not heat treated. Figure shows the result of oxidizing CO under O ₂ ). '2.0 H ₂ O' represents the result of oxidizing CO by contacting a Pd 2% by weight Pd / CeO ₂ catalyst without heat treatment to a CO and O ₂ stream comprising 3.2% H ₂ O.
FIG. 18 is a graph showing CO oxidation results under conditions with a slow space velocity of 24,000 ml / hrg _cat and a high O ₂ content of 10%. Hydrothermally treated Pd 2 wt% Pd / CeO ₂ catalyst was used for the 1% CO stream.
19 is a diagram showing in-situ DRIFT results of a hydrothermally treated Pd 2 wt% Pd / CeO ₂ catalyst. 2% CO and Ar were flowed at 25 ° C. for 20 minutes, the residual CO gas was purged under vacuum, and 2% O ₂ including Ar was flowed for 10 minutes while obtaining an IR spectrum at 25 to 125 ° C.
20 is a graph showing the change in catalytic activity for CO oxidation according to the amount of water included in the CO stream using Pd 2 wt% Pd / CeO ₂ catalyst without heat treatment.
21 (a) is a graph showing the results of CO oxidation by various hydrothermal treatment times, and (b) repeatedly reacted with a hydrothermally treated catalyst for 25 hours.
Figure 22 (a) is not heat-treated (as-made), CO in the case of using the hydrothermal-treated (HT), a post heat treatment at 570 ℃ hydrothermal treatment (570-HT) Pd / CeO of Pd 2% by weight of _second catalyst It is a graph showing the oxidation reaction rate. (b) is a graph showing the relationship between ln r and 1 / T from the reaction rate.
Figure 23 (a) is a diagram showing the result of the temperature desorption (TPD) method of Pd 2% by weight Pd / CeO ₂ catalyst hydrothermally treated with H ₂ ¹⁸ O. H ₂ ¹⁶ O (m / z = 18) and H ₂ ¹⁸ O (m / z = 20) were monitored by mass spectrometry. (b) is a diagram showing the results of Temperature-programmed reaction spectroscopy (TPRS) of Pd 2 wt% Pd / CeO ₂ catalyst hydrothermally treated with H ₂ ¹⁸ O. The CO flow was turned off and on again every 0.5 minutes. C ¹⁶ O ₂ (m / z = 44), C ¹⁶ O ¹⁸ O (m / z = 46), and C ¹⁸ O ₂ (m / z = 48) were monitored by mass spectrometry.
FIG. 24 (a) shows the result of co-oxidation of CO and C ₃ H ₆ using Pd / CeO ₂ catalyst of 2 wt% of hydrothermally treated (HT) Pd without heat treatment. (b) is the result of CO oxidation after contaminating the untreated heat treated (T), hydrothermally treated (HT) Pd 2 wt% Pd / CeO ₂ catalyst with sulfur (S).
Figure 25 (a) is a result of a long-term durability test for CO oxidation with a Pd / CeO ₂ catalyst of ₂ % by weight (T), hydrothermally treated (HT) Pd untreated, hydrothermally treated. (b) is a graph showing the results of CO oxidation in the presence of 100 ppm SO ₂ stream.
FIG. 26 is a graph showing the results of CO oxidation continuously at various temperatures using a hydrothermally treated Pd 2 wt% Pd / CeO ₂ catalyst. At the end of each CO oxidation cycle, the samples were cooled to room temperature and run at the next temperature.
FIG 27 (a) Pd / CeO of the non-heat-treated Pd 2% by weight of _second catalyst, was analyzed and (b) a hydrothermal treatment carbonate formation of the Pd 2 wt% Pd / CeO ₂ catalyst to the in-situ DRIFT, ( c) Pd / CeO ₂ catalyst with ₂ % by weight of Pd / CeO ₂ catalyst without heat treatment and (d) Pd / CeO ₂ catalyst with ₂ % by weight of hydrothermally treated Pd / CeO ₂ catalyst were analyzed by in-situ DRIFT.
28 (a) Cerium oxide not heat-treated, (b) Carbonate formation of hydrothermally treated cerium oxide was analyzed by in-situ DRIFT, (c) Cerium oxide not heat-treated, (d) Cerium oxide heat-treated In-situ DRIFT analyzes the presence of surface hydroxyl group in.
29 shows the surface of cerium oxide untreated and hydrothermally treated (HT) cerium oxide, untreated Pd 2 wt% Pd / CeO ₂ catalyst and hydrothermally treated (HT) Pd 2 wt% Pd / CeO ₂ catalyst surface After forming the carbonate, mass spectrometry was used to measure the TPD of CO ₂ (m / z = 44).
30 is a DRIFT data for the sulfate form of the Pd that is not 2 wt% Pd / CeO ₂ catalyst and the hydrothermal treatment (HT) of Pd 2% by weight Pd / CeO ₂ catalyst of the heat treatment. For comparison, DRIFT data of unheat treated cerium oxide, hydrothermally treated cerium oxide and Pd 2 wt.% Pd / Al ₂ O ₃ catalyst are shown.
(A) is a graph showing the TPD of oxygen (m / z = 32) observed by mass spectrometry of cerium oxide and Pd 2 wt% Pd / CeO ₂ catalyst before formation of surface carbonate and (b) after formation .
32 is a TPD graph when CO / He is flowed into an unheat-treated Pd 2 wt% Pd / CeO ₂ catalyst and a hydrothermally treated (HT) Pd 2 wt% Pd / CeO ₂ catalyst before and after carbonate formation (C) to be.

The present invention relates to a catalyst comprising a metal oxide nanoparticle and a noble metal nanoparticle fixed to the support, wherein the diameter of the noble metal nanoparticle is 0.3 to 1.6 nm, the catalyst for carbon monoxide oxidation. By hydrothermal treatment of the catalyst of the present invention, the diameter of the noble metal nanoparticles can be represented as in the above range, in which case it has a specific surface area of the catalyst that can exhibit the catalytic activity required for carbon monoxide oxidation.

In the present invention, the metal oxide may be cerium oxide or titanium oxide. Cerium oxide and titanium oxide exhibit strong metal-supporting interactions with noble metals and are suitable for use as a support for fixing catalysts comprising noble metal nanoparticles.

In the present invention, the noble metal may be selected from the group consisting of palladium, rhodium, platinum and ruthenium. Wherein the noble metal may be included in 0.1 to 5.0% by weight relative to the catalyst for carbon monoxide oxidation of the present invention. Preferably, it may be included in 2.0% by weight. When the content of the precious metal is less than 0.1% by weight, the carbon monoxide oxidation efficiency is significantly lowered, when higher than 5.0% by weight, the economic efficiency is lowered, the oxidation efficiency is lowered compared to the precious metal content.

In the present invention, the specific surface area of the catalyst may be 30 m ² / g or more. Preferably, the catalyst specific surface area may be between 30 and 60 m ² / g. When the specific surface area of the catalyst is lower than 30 m ² / g, there is a problem that the carbon monoxide oxidation efficiency is significantly lowered.

In the present invention, hydrothermal treatment collectively refers to a treatment performed through heating in a humid environment, and is distinguished from a heat treatment performed through heating in a dehumidified environment. In the hydrothermally treated carbon monoxide oxidation catalyst according to the present invention, the noble metal nanoparticles on the catalyst support may be redispersed. Here, the redispersion phenomenon of the noble metal nanoparticles according to the hydrothermal treatment, the proportion of the ionic noble metal nanoparticles increases as the ratio of the metallic noble metal nanoparticles is reduced, so that the cluster of the metallic noble metal nanoparticles is the ionic precious metal nano It may mean to be redispersed by the particles, but is not limited thereto. Redispersion of such noble metal nanoparticles may be to improve the catalytic activity of the catalyst for carbon monoxide oxidation according to the present invention. In addition, the catalyst for hydrothermally treated carbon monoxide oxidation according to the present invention can be suppressed the formation of carbonate species in the carbon monoxide oxidation process. Here, the carbonate species is a toxic component that results in reducing the carbon monoxide oxidation activity, the formation of the carbonate species by the hydrothermal treatment may be the result of the activity of the catalyst for the carbon monoxide oxidation is improved. . The catalyst for oxidizing carbon monoxide according to the present invention may be hydrothermally treated in air containing a temperature of 600 to 750 ° C. and 5 to 10% of H ₂ O. Hydrothermal treatment in air at temperatures below 600 ° C. or in air containing less than 5% H ₂ O results in poor noble metal nanoparticles redispersing the metal oxide support, leading to a sharp drop in carbon monoxide oxidation efficiency. . If the hydrothermal temperature exceeds 750 ℃, a problem that the noble metal nanoparticles are sintered occurs, and when hydrothermally treated in air containing more than 10% H ₂ O water is stuck to the surface of the catalyst too much to reduce the reactivity of the catalyst A problem arises. In addition, in the present invention, the hydrothermal treatment may be performed for 19 to 30 hours at the above temperature and humidity conditions. If the treatment is shorter than 19 hours, less redispersion of the metal nanoparticles occurs, and if the time exceeds 30 hours, the metal nanoparticles are sintered.

The present invention further relates to an automobile exhaust gas purification apparatus comprising a catalyst for oxidizing carbon monoxide according to the present invention, which is located at a rear end of an automobile internal combustion engine. Here, the automotive internal combustion engine includes, but is not limited to, a diesel or gasoline internal combustion engine.

In addition, the present invention is to form a support comprising a metal oxide nanoparticles, to fix a catalyst comprising a noble metal nanoparticles on the support to form a catalyst for carbon monoxide oxidation and hydrothermal treatment of the catalyst for carbon monoxide oxidation It relates to a method for producing a catalyst for carbon monoxide oxidation, including.

Example  And Experimental Example

Hereinafter, preferred examples are provided to aid in understanding the present invention. However, the following examples are merely provided to more easily understand the present invention, and the contents of the present invention are not limited by the examples.

Example 1-1. Pd / CeO ₂  Catalytic synthesis

The cerium oxide support was prepared by the precipitation method. A 0.1 M aqueous solution was prepared by dissolving 1000 mg of Ce (NO ₃ ) ₃ 6H ₂ O (canto chemical) in 23 mL of distilled water in a round-bottom flask. Thereafter, while the aqueous solution was continuously stirred (500 rpm), ammonia water (Duksan) was added dropwise until the pH of the solution reached 8.5 to form cerium hydroxide. After stirring for 1 hour at 500 rpm, a pale yellow slurry was obtained, filtered and washed several times with distilled water twice to remove anionic impurities. The precipitates obtained were dried in a convection oven at 80 ° C. for 12 hours, and the solids obtained were ground to fine powder in a ceramic mortar. The powder obtained was then calcined at 500 ° C. for 5 hours with a ramping rate of 5 ° C./min in an air atmosphere.

A palladium catalyst supported on cerium oxide was prepared by the deposition-precipitation method. 196 mg of cerium oxide powder and 5 mL of distilled water were added to a 20 mL vial, followed by vigorous stirring (900 rpm) to disperse the cerium oxide powder in distilled water. To prepare the H ₂ PdCl ₄ solution, a 2: 1 molar ratio of HCl (serial) and PdCl ₂ (99%, Sigma-Aldrich) were mixed in deionized water and sonicated until a homogeneous solution was formed. Separately, 530 mg of Na ₂ CO ₃ (99.5%, Sigma-Aldrich) was mixed with 10 mL of distilled water to prepare a Na ₂ CO ₃ aqueous solution. Thereafter, H ₂ PdCl ₄ (Pd 4 mg) and Na ₂ CO ₃ aqueous solution were introduced into the cerium oxide dispersion in a dropwise manner with stirring (900 rpm) to adjust the pH of the final solution to 9. After stirring for 2 hours and then standing at room temperature for 2 hours, the solution was filtered off and washed several times with deionized water and then dried at 80 ° C. for 5 hours.

Through the same method, Pd / CeO ₂ catalyst having 0.1 wt%, 0.2 wt%, 0.5 wt%, 1 wt%, 2 wt%, 4 wt% and 5 wt% Pd was synthesized.

Additionally, it was prepared by the the of 144.5 sccm air dried at a Pd / CeO ₂ catalyst prepared in the contact 750 ℃ for 25 hours heat treatment Pd / CeO ₂ catalyst.

In addition, by contacting of 144.5 sccm containing 10% H ₂ O to prepare a Pd / CeO ₂ catalyst at 750 ℃ air for 25 hours to prepare a hydrothermal treatment Pd / CeO ₂ catalyst.

Example 1-2. Rh / CeO ₂  Catalytic synthesis

A Rh 2 wt% Rh / CeO ₂ catalyst was synthesized by the method of Example 1-1, except that H ₂ RhCl ₅ solution was prepared and rhodium was used as the precious metal.

Example 1-3. Pt / CeO ₂  Catalytic synthesis

An H ₂ PtCl ₆ 6H ₂ O solution was prepared, and a Pt 2 Ce wt ₂ catalyst was prepared by the method of Example 1-1, except that platinum was used as the precious metal.

Example 1-4. Ru / CeO ₂  Catalytic synthesis

A Ru 2 wt% Ru / CeO ₂ catalyst was synthesized by the method of Example 1-1 except that H ₂ RuCl ₅ solution was prepared and ruthenium was used as the precious metal.

Comparative Example 1. Ir / CeO ₂  Catalytic synthesis

Ir 2 wt% Ir / CeO ₂ catalyst was synthesized by the method of Example 1-1 except that H ₂ IrCl ₅ solution was prepared and iridium was used.

Comparative Example 2. Au / CeO ₂  Catalytic synthesis

An Au 2 wt% Au / CeO ₂ catalyst was synthesized in the same manner as in Example 1-1 except that HAuCl ₄ 3H ₂ O solution was used to prepare gold.

Comparative Example 3 Cu / CeO ₂  Catalytic synthesis

Cu 2 wt% Cu / CeO ₂ catalyst was synthesized by the method of Example 1-1, except that H ₂ CuCl ₄ solution was prepared and copper was used.

Comparative Example 4. Co / CeO ₂  Catalytic synthesis

A Co 2 wt% Co / CeO ₂ catalyst was synthesized by the method of Example 1-1, except that cobalt was used to prepare a H ₂ CoCl ₄ solution.

Comparative Example 5. Ni / CeO ₂  Catalytic synthesis

A Ni 2 wt% Ni / CeO ₂ catalyst was synthesized by the method of Example 1-1, except that H ₂ NiCl ₄ solution was prepared and cobalt was used.

Example 2. Pd / TiO ₂  Catalytic synthesis

A Pd / TiO ₂ catalyst was synthesized in the same manner as in Example 1-1, except that titanium oxide was used as the support.

Comparative Example 6 Pd / ZrO ₂  Catalytic synthesis

A Pd / ZrO ₂ catalyst was synthesized by the method of Example 1-1 except that zirconium oxide was used as the support.

Comparative Example 7. Synthesis of Pd / MgO Catalyst

A Pd / MgO catalyst was synthesized by the method of Example 1-1, except that magnesium oxide was used as the support.

Comparative Example 8. Pd / Al ₂ O ₃  Catalytic synthesis

A Pd / Al ₂ O ₃ catalyst was synthesized by the method of Example 1-1 except that aluminum oxide was used as the support.

Comparative Example 9. Pd / Ce _0.5 Zr _0.5 O ₂  Catalytic synthesis

Jason AL et. al. Pd / Ce _0.5 Zr _0.5 O ₂ catalyst was synthesized using cerium oxide-zirconia as support and palladium as metal catalyst as described in (Appl. Cat. B. Env., 2016, 185, pp 189-202).

Experimental Example

CO Oxidation Reaction Using Catalyst

50 mg of each catalyst prepared according to Examples and Comparative Examples was injected into a U-type quartz cell. At this time, a simple prepared catalyst, a heat treated catalyst, and a hydrothermally treated catalyst without heat treatment were compared. The reactor was purged with 100 sccm of He gas at 100 ° C. for 1 hour and cooled to room temperature under He gas flow. A total of 100 sccm of feed gas containing 1% CO and 1% O ₂ was introduced for CO oxidation under He gas flow. The reactor was heated up at 25 ° C. to 10 ° C. at a rate of 5 ° C./min until it reached 100% CO conversion and held for 18 minutes to reach equilibrium after reaching each temperature.

When confirming the effect of the presence of propylene in the following was carried out in the same manner as above except that 100 sccm of the feed gas consisting of 1% CO, 0.2% C ₃ H ₆ and 10% O ₂ under He gas flow. . In addition, below, the influence of sulfur contamination was confirmed by the same method as above except that 100 ppm of SO ₂ / He flow was flowed at 100 ppm for 2 hours at 300 ° C on the catalyst. In some cases, water was flowed together with CO and O _2, and the water content was controlled by bubbling the inlet flow of the tank at various temperatures.

The resulting gas was subjected to a Kaboxen 1000 column (75035, SUPELCO, 15 ft × 1/8 in × 2.1 mm), on-line gas chromatography (GC, Younglin GC 6500 system) equipped with a thermal conductivity detector (TCD), capillary column (GS-GASPRO, Agilent Technologies, 30m × 0.32mm) and flame ionization detector (FID).

In-situ diffuse reflectance Fourier-transform infrared spectroscopy (DRIFT) measurements were performed using Nicolet iS-50 (Thermo Scientific). To this end, 13 mg of catalyst was mixed with 86 mg of KBr and ground, and the ground powder was injected into a sample cup and mounted in a DRIFT cell. The cell was covered with a KBr window and purged with 98 sccm of Ar gas for 20 minutes at room temperature. The cell was purged with an Ar flow at 100 ° C. for 1 hour to remove water and to cool to room temperature. The DRIFT cell was then heated to 85 ° C. for 20 minutes and the temperature was maintained at 85 ° C. because at this temperature the difference in activity between the prepared catalyst, untreated, heat treated, and hydrothermally treated catalysts was greatest. . Thereafter, 2% CO gas was charged at 85 ° C. for 10 minutes. DRIFT spectra were obtained in vacuo, and DRIFT spectra were simultaneously obtained after 2% O ₂ gas was charged at 85 ° C. for 10 minutes. Finally, additional spectra were obtained in vacuo.

How to check the characteristics of the catalyst

Transmission electron microscope (TEM) images were obtained using a Tecnai TF30 ST apparatus (300 kV). In addition, a high resolution transmission electron microscope (HR-TEM) image was obtained using TF30 ST (Tecnai, 300 kV acceleration voltage). Energy-dispersive X-ray spectroscopy (EDS) mapping data and high angle annular dark field-scanning TEM using titanium cubed G2 60-300 (FEI, 300 kV acceleration voltage); HAADF- STEM) image obtained. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis was performed on an OPTIMA 7300 DV instrument to determine the metal content in the catalyst.

The crystal structure of the catalysts was observed through X-ray diffraction (XRD) pattern (Rigaku, Cu Kα radiation). The state of the metal was confirmed using X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo VG Scientific). Binding energies were calculated using the maximum intensity of the favorable C 1s signal at 284.8 eV as a reference.

The diameter (d _x ) of the noble metal nanoparticles supported on the support was confirmed by a modified pulse CO adsorption technique developed by Takeguchi et al. The method utilizes pre-injection of CO ₂ to prevent overestimation of noble metal nanoparticle dispersion caused by the adsorption of CO on the support as carbonate species. First, about 50 mg of 2 wt% catalyst was heated at 300 ° C. (10 ° C./min) for 10 minutes in 3.5% O ₂ / He gas, then cooled to 50 ° C., and the sample was 4.9% H _2. Heated to 200 ° C. in a / Ar flow and cooled to 50 ° C. The sample was then exposed to the gas stream in the following order: (i) He (5 minutes); (ii) 3.5% O ₂ / He (5 minutes); (iii) CO ₂ (10 minutes); (iv) He (20 minutes); (v) 5% H ₂ / Ar (5 minutes). Finally, CO was pulsed every minute in the He stream until the adsorption of CO to the sample was saturated.

The surface carbonate formed from the catalyst was measured by a temperature desorption method (TPD) on BELCAT-B (BEL, Japan) equipped with a mass spectrometer (MS). 50 mg of each catalyst was placed in a U-type quartz cell and pretreated at 100 ° C. for 1 hour under a 100 flow flow of He and cooled to room temperature. 1% CO and 1% O ₂ were introduced into the cell for 20 minutes at room temperature under He gas flow. After purging physically adsorbed gas molecules with a flow of 100 sccm for 1 hour, the catalyst was immediately charged into a TPD-quartz cell. The TPD-quartz cell was heated to 600 ° C. at a rate of 10 ° C./min when the He flow was 50 sccm. Carbon dioxide with m / z = 44 in the vented gas was analyzed by MS detector. The availability of lattice oxygen was measured by TPD-MS by monitoring oxygen. Samples were pretreated with 50 sccm of 3.5% O ₂ / He at 300 ° C. for 1 hour and cooled to room temperature under 50 sccm of He flow. The temperature was raised to 800 ° C. at a rate of 10 ° C./min under He flow. Oxygen with m / z = 32 in the vented gas was analyzed by MS detector.

Confirmation of diameter change of metal by hydrothermal treatment

Electron micrographs of an unannealed Pd / CeO ₂ catalyst, a heat treated Pd / CeO ₂ catalyst and a hydrothermally treated Pd / CeO ₂ catalyst sample prepared according to Example 1 are shown in FIG. 2. The approximate diameter of cerium oxide was about 15-30 nm as shown in FIG. 3. When 2% by weight of Pd was deposited on the cerium oxide surface, very small Pd nanoparticles were observed as indicated by the white circles in Fig. 2 (a). When the prepared catalyst was heat treated at 750 ° C. for 25 hours in dry air, it was confirmed that the Pd domain was significantly increased. However, little Pd domain was observed when the prepared catalyst was heat-treated at 750 ° C. for 25 hours in humid air of 10% H ₂ O. The EDS mapping image of the hydrothermally treated sample in FIG. 2 (d) clearly shows that Pd is uniformly distributed on cerium oxide, but Pd nanoparticles were not distinguished by HAADF-STEM. The EDS mapping image of the prepared Pd / CeO ₂ catalyst and the heat treated Pd / CeO ₂ catalyst sample showed that Pd forms separate domains as shown in FIG. 4. The high resolution TEM images of FIGS. 5-7 show 2 wt% Pd / CeO ₂ The average diameter of the Pd domain was 1.5 nm for the catalyst, 2.2 nm for the heat treated samples and 0.9 nm for the hydrothermally treated samples. These results demonstrate that the Pd domain becomes large after the heat treatment and the Pd domain becomes small after the hydrothermal treatment.

The reduction of the diameter of the Pd domain after hydrothermal treatment, also called redispersion, could be confirmed by chemical adsorption of CO. The BET surface area, Pd dispersion and estimated diameter, oxidation state of Pd and changes in T ₁₀₀ of the catalyst samples are shown in Table 1.

catalyst S _BET (m ² / g) D _pd (%) ^a d _pd (nm) ^b Pd ⁰ /
(Pd ⁰ + Pd ^{2 +} ) ^c T ₁₀₀ (℃) ^d Cerium oxide 58.7 - - - 270 Heat treated cerium oxide 4.6 - - - 300 Hydrothermally treated cerium oxide 4.9 - - - 290 2% Pd / CeO _{2 prepared} 57.4 59.1 1.5 0.40 125 2% by weight of Pd / CeO ₂ heat-treated 38.5 36.7 2.4 0.22 145 2 wt% Pd / CeO ₂ hydrothermally treated 39.9 97.8 0.9 0.14 75

a: Pd dispersion was measured by pulsed CO chemisorption.

b: The diameter of Pd particle | grains was estimated from Pd dispersion.

c: Pd 3d XPS spectra were estimated.

d: represents the temperature at which the CO conversion reached 100%.

As shown in Table 1, the changes in BET surface area and Pd oxidation state were measured to confirm the physicochemical properties of the Pd / CeO ₂ catalyst by hydrothermal treatment. If the specific surface area of cerium oxide showed the agglomeration tendency and reduced to 4.9m ² / g from 58.7m ² / g after hydrothermal treatment, hydrothermal treatment for 2 wt% Pd / CeO ₂ catalyst, at 39.9m 57.4m ² / g ^It decreased to ² / g, and it was confirmed that cerium oxide aggregation is suppressed with addition of Pd. After hydrothermal treatment, Pd dispersion increased to 97.8%, and the diameter of Pd nanoparticles decreased to 0.9 nm, indicating that redispersion of Pd nanoparticles occurred. The change of Pd oxidation state after hydrothermal treatment was estimated as shown in FIG. 8 using XPS. It was confirmed that Pd was further oxidized after hydrothermal treatment. The ratio of Pd ⁰ to Pd ⁰⁺ + Pd ²⁺ estimated from XPS data decreased significantly after hydrothermal treatment. The change in crystal structure was also observed in XRD as shown in FIG. 9. An XRD peak for PdO was observed in the Pd / CeO ₂ catalyst at Pd 2 wt%, but after hydrothermal treatment the PdO peak disappeared and the redispersion of the Pd domain was also confirmed.

Determine the effect of hydrothermal treatment on the catalyst

The effect of hydrothermal treatment conditions on Pd / CeO ₂ catalyst at 2 wt% Pd was confirmed. Pd domains were estimated as shown in Table 2 by varying the moisture content, temperature, and treatment time. As the water content increased by 10%, the average Pd diameter decreased. In addition, the temperature of 750 ° C. and hydrothermal treatment for 25 hours were the optimum treatment conditions for Pd redispersion supported on cerium oxide. The CO oxidation activity of hydrothermally treated Pd / CeO ₂ under various conditions is shown in FIG. 10. In the water content experiments, the catalytic activity increased up to 10%, but as the water content increased, the excess of water blocked the active sites, which greatly reduced the catalytic activity. Even in experiments with different temperatures and times, the activity of the more redispersed catalyst was better, which means that the Pd redispersion strategy is effective in improving the CO oxidation reaction activity.

H ₂ O content (%) Unheated catalyst 3 5 10 15 20 30 Pd diameter after hydrothermal treatment (nm) 1.5 1.9 1.3 0.9 1.0 0.9 1.0 Hydrothermal treatment temperature (℃) Unheated catalyst 300 450 600 750 850 950 Pd diameter after hydrothermal treatment (nm) 1.5 1.7 2.1 0.9 0.9 3.6 7.4 Hydrothermal treatment time (h) Unheated catalyst 12 25 50 - - - Pd diameter after hydrothermal treatment (nm) 1.5 1.2 0.9 3.4 - - -

In order to further identify the Pd redispersion phenomenon, the change in size of the Pd domain with respect to hydrothermal treatment time is shown in Table 3. As a result, it was confirmed that the Pd domain grew from 1.5 nm to 2 nm and then gradually decreased by 4 hours. Hydrothermal treatment can provide surface -OH groups to form Pd ²⁺ -OH. The amount of surface -OH groups is insufficient up to 4 hours, but hydrothermal treatment longer than 4 hours provides a sufficient amount of -OH groups to promote Pd redispersion. That is, the formed Pd ^{2 +} -OH species move with high mobility, and at this time, Pd supported on the cerium oxide is redispersed due to the strong interaction between Pd ^{2 +} -OH and cerium oxide.

Hydrothermal treatment time (h) 0 0.5 One 2 3 4 5 6 7 Pd diameter (nm) 1.5 1.5 1.6 1.8 1.9 2.0 2.0 2.0 1.9 Hydrothermal treatment time (h) 8 9 10 11 12 13 14 15 16 Pd diameter (nm) 1.7 1.6 1.4 1.3 1.2 1.2 1.2 1.1 1.1 Hydrothermal treatment time (h) 17 18 19 20 21 22 23 24 25 Pd diameter (nm) 1.1 1.1 1.0 1.0 1.0 1.0 0.9 0.9 0.9

Improved metal-support interactions after hydrothermal treatment were confirmed via XANES and EXAFS. The change of Pd oxidation state after hydrothermal treatment was evaluated using XANES as shown in FIG. 11. XANES data showed that the white line peaks of hydrothermally treated Pd / CeO ₂ shifted to higher energy than untreated heat treated catalyst samples and heat treated samples. The highly oxidized Pd in hydrothermally sampled is probably due to the generation of Pd ²⁺ -OH. In addition, the Pd K edge EXAFS data was obtained by Pd 2 wt% untreated Pd / CeO ₂ catalyst sample, heat treated Pd / CeO ₂ catalyst sample, hydrothermally treated Pd / CeO ₂ catalyst sample, PdO and Pd as shown in FIG. The foil samples were compared and the fitting results are shown in Table 4. As a result, the coordination number of Pd-O-Pd increased from 3.5 to 5.2 after heat treatment. However, the coordination number of Pd-O-Pd decreased from 3.5 to 0.6 after hydrothermal treatment, and the coordination number of Pd-O-Ce increased from 0.3 to 1.7. Therefore, hydrothermal treatment can be seen to enhance the interaction of Pd-O-Ce while reducing the Pd domain size.

Sample pass Coordination Number (R) Interatomic distance Divider-Waller Factor (σ ² / Å ² ) R-factor (%) Not heat-treated
Not sample Pd-O 2.236 1.979 0.001 0.015 Pd-Pd 0.113 2.681 0.003 Pd-Cl 1.872 2.290 0.006 Pd-O-Ce 0.321 3.214 0.003 Pd-O-Pd 3.491 3.354 0.015 Heat-treated
Pd 2.0 wt% Pd / CeO ₂ Pd-O 4.571 2.003 0.003 0.018 Pd-Pd 0.453 2.739 0.003 Pd-O-Ce 0.429 3.400 0.001 Pd-O-Pd 5.236 3.360 0.001 Hydrothermal
Pd 2.0 wt% Pd / CeO ₂ Pd-O 4.575 1.998 0.003 0.026 Pd-Pd 0.122 2.705 0.003 Pd-O-Ce 1.678 3.215 0.004 Pd-O-Pd 0.585 3.407 0.003 PdO Pd-O 4.0 2.024 0.004 0.005 Pd-O-Pd 4.0 3.060 0.007 Pd-O-Pd 8.0 3.443 0.005 Pd foil Pd-Pd 12.0 2.742 0.006 0.003

Adsorption of CO at the metal reaction site and in-situ IR analysis provided important information about the metal structure. CO molecules adsorbed linearly at a single Pd site peak at 2000-2200 cm ⁻¹ , while CO adsorbed at Pd ensemble sites with bridge mode or 3-fold hollow mode is 1900-2000 cm ⁻¹ Or a peak appeared at 1800 to 1900 cm ⁻¹ . FIG. 13 shows the IR peaks of CO adsorbed on unannealed catalyst, heat treated catalyst and hydrothermally treated Pd / CeO ₂ samples. The heat treated sample showed a large hollow CO peak due to Pd aggregation. However, in the hydrothermally treated samples, the bridge or hollow CO peaks were almost gone, and the linear CO peaks increased significantly to support Pd redispersion. XANES data, EXAFS data, in-situ IR data, it may be formed with an oxidized Pd ^{2 +} -OH, metal-support interactions, indicating that it is because the enhanced Pd / CeO ₂ catalyst Pd domains on the redistribution during the hydrothermal treatment.

To explore Pd redispersion in depth, the prepared Pd / CeO ₂ was heat-treated under He flow to control the initial Pd diameter of 1.5 to 17.8 nm, and then Pd 2 wt% Pd / CeO ₂ catalysts of various diameters were listed. Hydrothermal treatment, as shown in Figure 5, observed redispersion of the initial Pd domain size. Pd nanoparticles with diameters of 1.5, 1.6 and 2.3 nm showed nearly 100% dispersion and were completely redispersed, while larger diameter Pd nanoparticles had a significant reduction in Pd domain size, but not fully redispersion. Redispersion may occur repeatedly as shown in Table 6. The redispersed Pd sample was sintered through heat treatment, and then redispersed by hydrothermal treatment. Redispersion occurred reversibly up to 13 times, and after hydrothermal treatment, all the Pd was completely redispersed. Thus, hydrothermal treatment methods can be repeatedly applied to regeneration processes through redispersion of sintered Pd metal catalysts in practical industrial applications.

Heat treatment temperature (℃) Room temperature 200 500 550 600 650 700 800 Initial Pd Dispersion (%) 59.3 55.5 38.7 30.7 23.4 15.6 10.5 5.0 Initial Pd Diameter (nm) 1.5 1.6 2.3 2.9 3.8 5.7 8.5 17.8 Pd dispersion after heat treatment (%) 97.9 98.7 98.3 74.2 37.1 19.4 11.6 4.9 Pd diameter after heat treatment (nm) 0.9 0.9 0.9 1.2 2.4 4.6 7.7 18.3

The initial diameter of the Pd nanoparticles was controlled by annealing for 2 hours under 100 sccm of He at various temperatures before hydrothermal treatment.

cycle Before heat treatment First round R First round S Round two R Second round S Third round R Third round S Fourth round R Fourth round S Pd diameter (nm) 1.5 0.9 2.1 1.0 2.1 0.9 2.2 0.9 2.2 cycle 5th R 5th S 6th round R Sixth round S Seventh round R Seventh round S 8th R 8th S 9th R Pd diameter (nm) 1.0 2.2 0.9 2.3 0.9 2.3 0.9 2.3 1.0 cycle 9th S 10th R 10th S Round 11 R 11th round S Round 12 R 12th round S 13th round R Pd diameter (nm) 2.3 0.9 2.2 1.0 2.3 0.9 2.3 0.9

'R' means redispersed by hydrothermal treatment at 750 ° C. for 25 hours using 10% H ₂ O flow, and 'S' is sintered by heat annealing at 500 ° C. for 2 hours using He flow. it means.

Identification of metal redispersion according to the type of catalyst support

Pd redispersion supported by cerium oxide through hydrothermal treatment was confirmed through various experiments, and it was confirmed whether Pd redispersion occurred in other metal oxide supports as given in Table 7. As a result, Pd was redispersed after hydrothermal treatment even on the TiO ₂ support. On the other hand, Pd nanoparticles were significantly sintered in ZrO ₂ , MgO, and Al ₂ O ₃ supports. CeO ₂ and TiO ₂ supports are known to interact strongly with Pd. Therefore, strong metal-supporter interaction is essential to cause Pd redispersion through hydrothermal treatment.

catalyst
(Pd 2 wt%) % Dispersion of unheated catalyst Pd diameter of unheated catalyst (nm) Dispersion of Hydrothermally Treated Catalysts (%) Pd diameter (nm) of hydrothermally treated catalyst Pd / CeO ₂ 59.1 1.5 97.8 0.9 Pd / TiO ₂ 30.6 2.9 74.4 1.2 Pd / ZrO ₂ 31.7 2.8 6.8 13.1 Pd / MgO 19.7 4.5 6.0 14.7 Pd / Al ₂ O ₃ 26.1 3.4 1.6 55.5

In addition, the CO oxidation activity was confirmed using the catalysts of Example 1 and Comparative Example 9. Pd used a catalyst contained in 2% by weight. In Figure 14, "Ref lean" means that the catalyst of Comparative Example 9 was flowed 0.1% O ₂ , 10% H ₂ O at 700 ℃, N ₂ conditions. The reaction condition is that 50 mg of each catalyst was brought into contact with 1% CO and 1% O ₂ , SV = 120,000 ml / g _cat.h under He flow.

In the case of automotive exhaust purification catalysts, it is very important to remove all CO below 150 ° C. However, in the case of using the catalyst of Comparative Example 9 it can be seen that only CO must be removed when all the CO is removed. Jason. AL et. In al., it was confirmed that Pd was redispersed even by the dry lean treatment after the Rich treatment using the catalyst of Comparative Example 9. When the catalyst of Comparative Example 9 is used, PdO plays an important role in Pd redispersion. In contrast, in the case of the catalyst of Example 1, Pd redispersion occurred only when hydrothermally treated. This means that Pd ²⁺ -OH played an important role in redispersion. In the case of the present invention, the catalytic activity effect of hydrothermal treatment contributes not only to Pd redispersion but also to the formation of surface hydroxyl groups.

Identify redispersion by metal type

After the various transition metals were supported on the cerium oxide support, the synthesized M / CeO ₂ catalyst was hydrothermally treated and summarized in Table 8. Among the various transition metals, Pd, Rh, Pt, and Ru were redispersed after hydrothermal treatment. However, Ir, Au, Cu, Co, Ni metals were considerably sintered after hydrothermal treatment. During hydrothermal treatment, the metal species is converted to M ²⁺ -OH form. In addition, many OH groups are formed on the surface of cerium oxide. For metal redispersion, M ²⁺ -OH species must be more mobile than metal or metal oxide, and the interaction between metal and support should be stronger than that of cerium oxide with OH formed on the surface. In other words, Pd, Rh, Pt and Ru correspond to this theory and cause metal redispersion after hydrothermal treatment.

catalyst
(2% by weight of metal) % Dispersion of unheated catalyst Metal diameter of untreated catalyst in nm Dispersion of Hydrothermally Treated Catalysts (%) Metal diameter of hydrotreated catalyst (nm) Pd / CeO ₂ 59.1 1.5 97.8 0.9 Rh / CeO ₂ 38.7 2.3 89.7 1.0 Pt / CeO ₂ 35.5 2.5 73.3 1.2 Ru / CeO ₂ 42.2 2.1 56.4 1.6 Ir / CeO ₂ 32.9 2.7 5.5 15.9 Au / CeO ₂ ≥100 ≤0.9 3.7 23.6 Cu / CeO ₂ 21.1 4.2 2.7 32.0 Co / CeO ₂ 17.4 5.1 2.0 43.5 Ni / CeO ₂ 26.9 3.3 1.7 52.7

All metals (Pd, Pt, Rh, Ru) showed higher CO oxidation activity after hydrothermal treatment due to metal redispersion. Metal redispersion provides more catalytic reaction sites than the initial sample, further improving the CO oxidation activity. Catalytic activity for the CO oxidation reaction was found in the order Pd> Pt> Rh> Ru (FIG. 15).

Catalyst activity and durability against CO oxidation

The catalytic activity for CO oxidation of Pd / CeO ₂ catalysts having various Pd contents was shown in FIG. 16. Although cerium oxide showed little activity up to 200 ° C., the activity was improved even by adding 0.1% by weight of Pd. Catalyst activity increased as the Pd content of the catalyst sample increased up to 5% by weight. In the case of hydrothermally treated catalyst samples, the activity was increased by 2% by weight, and further increase in Pd content showed no significant difference in activity. Larger diameters (4.0% to 5.0% by weight) of Pd nanoparticles rarely occurred redispersion. Total redispersion was only observed up to 2% by weight. The catalytic activity for CO oxidation increased to 2% by weight of Pd content. The activity and durability were further examined using a catalyst having a Pd content of 2% by weight.

The temperature at which the CO conversion reaches 100% (hereinafter referred to as T ₁₀₀ ) is determined by Pd 2 wt% Pd / CeO ₂ catalyst prepared according to Example 1 at 125 ° C. and Pd 2 wt% Pd / CeO ₂ heat treated. The catalyst was 145 ° C., and the hydrothermally treated Pd 2 wt% Pd / CeO ₂ catalyst was found to be 75 ° C. (FIG. 17). Hydrothermally treated samples showed improved catalytic activity despite the reduction in surface area due to Pd redispersion. The catalytic activity of the heat-treated sample was lower due to the Pd sintering phenomenon, and after hydrothermal treatment, it was confirmed that the reactivity was improved as the Pd nanoparticles were redispersed. As shown in FIG. 18, when the space velocity is slow or the composition of oxygen is increased, the hydrothermally treated sample reaches 100% of CO conversion even at 40 ° C., which corresponds to a very low temperature.

In situ DRIFT analysis was performed to reveal the actual active site for CO oxidation at low temperature. Figure 19 shows that Pd ^{2 +} site is causing the CO oxidation at low temperature. Peak of 2074 cm ^-1 and 2106 will represent a linear CO adsorbed on Pd ⁰ sites, the peak at 2152 cm ^-1 is shown a linear CO adsorbed on Pd ^{+ 2} position. CO-Pd ^{2 +} peak disappeared from between 25 ℃ disappeared between 75 ℃, CO-Pd ⁰ peak is from 65 ℃ 125 ℃. This means that CO oxidation activity at low temperatures occurs mainly at the redispersed Pd ²⁺ site.

When water was introduced together with CO and O ₂ , the activity of the prepared Pd 2 wt% Pd / CeO ₂ catalyst was improved, but not as much as the hydrothermally treated catalyst sample. It has been reported that the addition of water to the conventional feed gas stream can significantly improve the activity of the metal catalyst. The water content in the feed stream was checked by varying up to 10% as shown in FIG. 20. The catalytic activity was highest when the water content was 3.2% and the activity decreased as the water content increased. The addition of water has the effect of promoting CO oxidation by increasing the amount of hydroxyl groups on the catalyst surface. Hydrothermally treated catalysts exhibit the best activity due to the combined effect of Pd redispersion and CO oxidation due to hydroxyl formation on the catalyst surface.

Hydrothermal treatment time was confirmed by various adjustments as shown in Figure 21 (a). The catalyst samples according to Example 1 were hydrothermally treated for 12 to 50 hours each. 25 hours of hydrothermal treatment showed the best activity. The BET surface areas after hydrothermal treatment were 47.2, 39.9 and 13.3 m ² / g after 12 hours, 25 hours and 50 hours, respectively. 12 hours may not be sufficient for Pd redispersion and after 50 hours treatment the surface area has decreased too much. CO oxidation was repeated five times for a 25 hour hydrothermally treated catalyst as shown in FIG. 21 (b) but no change in activity was observed.

While the Pd domain size was the same as Pd / CeO ₂ without heat treatment, a Pd / CeO ₂ catalyst was prepared with more surface hydroxyl groups to independently observe the effect of Pd redispersion and surface hydroxyl groups on CO oxidation. The catalyst sample prepared according to Example 1 was heat treated at 570 ° C. with a He flow for 2 hours and then hydrothermally treated at 750 ° C. for 25 hours (denoted 570-HT), where the Pd domain size was 1.5 nm and more surface hydroxyl groups. It will contain. 22 shows the CO oxidation results for this catalyst. The activity of this catalyst was higher than that of the sample without heat treatment, but lower than that of the hydrothermally treated sample. This means that both Pd redispersion and surface hydroxyl groups play a role in enhancing CO oxidation activity. The activation energies of the unannealed catalyst, 570-HT and hydrothermally treated samples were 57.0, 53.8 and 53.5 kJ / mol, respectively. As such, surface hydroxyls play an important role in reducing activation energy. FIG. 23 shows whether surface hydroxyl groups are directly involved in carbon monoxide oxidation by isotope experiments. Hydrothermal treatment was performed with H ₂ ¹⁸ O. It was confirmed that ¹⁸ OH was formed on the surface by the TPD shown in FIG. 23 (a). Thereafter, CO oxidation was performed on the catalyst as shown in FIG. 23 (b). Only C ¹⁶ O ₂ was observed without C ¹⁶ O ¹⁸ O or C ¹⁸ O ₂ , indicating that oxygen contained in the surface hydroxyl groups was not directly involved in CO oxidation.

In order to evaluate the potential of the hydrothermally treated sample as a diesel oxidation catalyst, CO and C ₃ H ₆ were simultaneously added to the reactor, and the reaction results are shown in FIG. In the presence of C ₃ H ₆ , T ₁₀₀ increased from 75 ° C. to 145 ° C. Nevertheless, it was found that CO and C ₃ H ₆ were fully oxidized at 145 and 155 ° C, respectively, to almost meet the DOE target for the low-emission automotive program LEV III.

Sulfur contamination is a very important issue for diesel oxidation catalysts because sulfur contained in automotive exhaust gases can contaminate the catalyst surface and degrade catalyst activity. In particular, cerium oxide is known to be contaminated with sulfur well. Before carrying out the CO oxidation reaction with Pd 2 wt.% Pd / CeO ₂ catalyst, 100 ppm SO ₂ was deliberately flowed into the source for 2 hours in advance. At this time, CO conversion is shown in FIG. 24 (b). T ₁₀₀ increased from 125 ° C. to 235 ° C. in the unheated sample, from 145 ° C. to 285 ° C. for the heat treated sample and from 75 ° C. to 145 ° C. for the hydrothermally treated sample. The T ₁₀₀ increase was the smallest for the hydrothermally sampled and all CO was completely oxidized below 150 ° C even in the presence of sulfur. As such, it was found that hydrothermal treatment significantly increased the sulfur resistance of the Pd / CeO ₂ catalyst.

The long term stability of the catalyst was tested by prolonged oxidation of CO at 150 ° C. with Pd 2 wt.% Pd / CeO ₂ catalyst. The results are shown in Figure 25 (a). The catalyst activity began to decrease after 27 hours for the unannealed catalyst sample and after 8 hours for the heat treated catalyst sample, while the hydrothermally treated catalyst sample maintained 100% CO conversion for 34 hours. The activity of the heat treated samples was significantly reduced, and the hydrothermally treated samples showed similar trends up to 60 hours before deactivating. When CO is oxidized for a long time, most of the hydroxyl groups on the surface of the catalyst are removed, nevertheless, the durability of the hydrothermally treated samples is improved upon CO oxidation in the presence of 3.2% water. As shown in FIG. 25 (b), 100% CO conversion of the hydrothermally treated sample was maintained for 14 hours when 100 ppm SO _{2 was} simultaneously flowed together with CO and O ₂ , even when the conversion rate was changed to 80%. It was confirmed that high CO conversion was maintained up to 60 hours. When the non-heat treated catalyst sample was subjected to CO oxidation in the presence of SO ₂ , the catalyst activity dropped significantly from the initial stage, and the conversion rate became 0% after 35 hours. As such, hydrothermal treatment was found to help maintain high catalytic activity even in the presence of sulfur.

In addition, the durability according to the temperature change was tested using a hydrothermally treated Pd 2% by weight Pd / CeO ₂ catalyst. As shown in FIG. 26, CO oxidation was repeated at various temperatures of 75 to 850 ° C. CO oxidation was carried out to one target temperature and the reactor was cooled to room temperature and then run again at the next target temperature. Hydrothermally treated samples showed high durability despite rapid temperature changes and little change in activity of CO oxidation.

Surface carbonate and lattice oxygen

Carbon dioxide is formed on the surface of the catalyst during CO oxidation, and this carbonate is well known as a contaminant that degrades catalytic activity. The degree of carbonate formation was confirmed using in-situ DRIFT (Figures 27 (a) and (b)). It was found that the carbonate was formed in the form of bidentate carbonates (1585 and 1299 cm ⁻¹ ) or monodentate carbonates (1482 and 1416 cm ⁻¹ ). The infrared peak intensity of carbonate was found to decrease dramatically after hydrothermal treatment. 27 (c) and (d) show that the hydrothermally treated sample contains much more hydroxyl groups than the unheated sample. The hydroxyl groups on the surface of the catalyst can interfere with the formation of carbonates on the surface. As shown in FIG. 28, even when in-situ DRIFT analysis was performed on the cerium oxide, the same tendency as the Pd / CeO ₂ catalyst was observed.

Carbon dioxide formed on the catalyst surface during CO oxidation was analyzed using CO ₂ elevated desorption (TPD) (FIG. 29). Reaction gases containing CO and O ₂ were introduced together under the same conditions as CO oxidation of Experimental Example 1, purged with He to remove the gas physically adsorbed on the catalyst, and then the temperature was detected while detecting the desorbed CO ₂ by mass spectrometry. Increased. The catalyst which was not heat-treated produced a large amount of carbonate on the surface and showed a CO ₂ desorption peak at 250, 410, and 550 ° C., but the hydrothermally treated catalyst suppressed carbonate formation and hardly detected a CO ₂ peak. Therefore, it was confirmed reliably that hydrothermal treatment suppressed carbonate formation on the surface.

Sulfur can form sulfates on the surface of the catalyst to deactivate the metal catalyst supported on the support. The unannealed or hydrothermally treated Pd / CeO ₂ catalyst was intentionally contaminated with 100 ppm SO ₂ and then the surface sulphate formed was observed using DRIFT (FIG. 30). The intensity of the sulfate IR peak was shown to decrease after hydrothermal treatment. The cerium oxide or the hydrothermally treated cerium oxide which did not support the metal was also contaminated with 100 ppm SO ₂ , and the IR peak was confirmed, and it was confirmed that the intensity of the sulfate peak decreased significantly after the hydrothermal treatment. When the hydrothermally treated Pd 2 wt% Pd / Al ₂ O ₃ was contaminated with sulfur, no sulfate peak was observed. These results show that sulfate is formed mainly on the surface of cerium oxide, and that sulfate formation on the surface of cerium oxide is significantly suppressed by hydrothermal treatment.

Hydroxyl groups and carbonates on the surface can affect the transport of oxygen. FIG. 31 shows O ₂ -TPD results when carbonate is present or absent on the surface of cerium oxide and a Pd / CeO ₂ catalyst at ₂ % by weight of Pd. All samples were pretreated under O ₂ flow at 300 ° C. for 1 hour. Compared with cerium oxide, the addition of Pd showed a larger O ₂ peak. In the case of Pd / CeO ₂ catalyst, a surface lattice oxygen peak at 260 ° C. and a bulk lattice oxygen desorption peak at 530 ° C. were observed by hydrothermal treatment to generate hydroxyl groups on the catalyst surface. On the other hand, the catalyst sample without heat treatment showed a desorption peak of surface lattice oxygen at 300 ° C. and bulk lattice oxygen at 550 ° C. Surface carbonate was formed while flowing CO and O ₂ at room temperature, and O ₂ -TPD was performed. The untreated heat treated catalyst sample did not show an O ₂ desorption peak, but the hydrothermally treated catalyst sample still showed large O ₂ peaks for surface lattice oxygen at 270 ° C. and bulk lattice oxygen at 520 ° C. As such, the transfer of oxygen was hindered by the surface carbonate in the untreated heat treated catalyst sample, but the hydrothermally treated catalyst sample was hardly affected due to the inhibition of carbonate formation.

TPD analysis was performed on the CO / He flow and is shown in FIG. 32. Hydrothermally treated Pd / CeO ₂ showed a large peak at 83 ° C regardless of carbonate formation. However, the untreated heat treated catalyst sample showed a CO ₂ peak at 129 ° C only in the uncontaminated state, and the carbonate formed sample showed little CO ₂ peak. The amount of desorbed CO ₂ was estimated as shown in Table 9. The hydrothermally produced catalyst produced a large amount of carbon dioxide, but the amount of carbon dioxide was significantly reduced after carbonate formation in the unannealed catalyst sample. As such, the hydrothermal treatment was found to promote the transport of lattice oxygen and interfere with carbonate formation, resulting in high activity and durability against CO oxidation.

Catalyst (Pd 2 wt%) ^b Main CO ₂ peak temperature (℃) Amount of adsorbed CO ₂ (mmol / g _cat ) ^c 2.0 HT 83 0.183 2.0 HT C 83 0.164 2.0 129 0.157 2.0 C - 0.028

Table 9 Confirmation conditions: catalyst: 50 mg, pretreatment: 100 sccm He for 1 hour at 100 ° C., TPD analysis: balanced with 10% CO and He of 50 sccm, rising rate of 10 ° C./min.

b: 'HT' indicates hydrothermal treatment and 'C' indicates that carbonate was formed on the surface.

c: The amount of desorbed CO ₂ was estimated using CO ₂ correction data and TCD signal region.

As described above, the present invention has been described through examples. Those skilled in the art will appreciate that the present invention can be implemented in other specific forms without changing the technical spirit or essential features. Therefore, the above-described embodiments are to be understood as illustrative in all respects and not restrictive. The scope of the present invention is shown by the following claims rather than the detailed description, and all changes or modifications derived from the meaning and scope of the claims and equivalent concepts should be construed as being included in the scope of the present invention.

Claims (12)

A support comprising cerium oxide or titanium oxide nanoparticles; And
It includes; precious metal nanoparticles fixed to the support;
The precious metal is included in 0.1 to 5.0% by weight relative to the total weight
The precious metal nanoparticles are 0.3 to 1.6 nm in diameter,
The precious metal is one or more selected from the group consisting of palladium, rhodium, platinum and ruthenium
Catalyst for Oxidation of Carbon Monoxide. delete delete delete The method of claim 1,
A catalyst for carbon monoxide oxidation, wherein the specific surface area of the catalyst is 30 to 60 m ² / g. A vehicle exhaust gas purifying apparatus comprising the catalyst for oxidizing carbon monoxide according to claim 1. 1) forming a support comprising cerium oxide or titanium oxide nanoparticles;
2) calcining the support;
3) fixing a noble metal nanoparticle on the support of step 2) to form a catalyst for carbon monoxide oxidation,
The precious metal is selected from the group consisting of palladium, rhodium, platinum and ruthenium, and the precious metal is included in an amount of 0.1 to 5.0 wt% based on the total weight; And
4) hydrothermally treating the catalyst for oxidizing carbon monoxide in step 3), wherein the hydrothermal treatment is performed for 19 to 30 hours;
As a method for producing a catalyst for oxidizing carbon monoxide,
The diameter of the noble metal nanoparticles of the catalyst for oxidizing carbon monoxide prepared by the production method is 0.3 to 1.6 nm
Method for producing a catalyst for oxidizing carbon monoxide. delete delete The method of claim 7, wherein
The hydrothermal treatment is carried out at a temperature of 600 to 750 ℃, method for producing a catalyst for carbon monoxide oxidation. The method of claim 7, wherein
The hydrothermal treatment is carried out in air containing 5 to 10% H ₂ O, the method for producing a catalyst for carbon monoxide oxidation. delete

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