KR20180072541A - 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 hydrothermally treated catalyst for the carbon monoxide oxidation which contains a catalyst including a support including metal oxide nanoparticles and noble metal nanoparticles fixed to the support; and a manufacturing method thereof. The catalyst according to the present invention solves the problem of catalyst inactivation by applying hydrothermal treatment conditions which are substantially similar to the actual vehicle exhaust environment, thereby contributing to the improvement of catalytic activity and the design of a stable automobile catalyst.

Description

FIELD OF THE INVENTION The present invention relates to a hydrotreated carbon monoxide oxidation catalyst comprising a metal oxide support and noble metal nanoparticles fixed on the support, a method for producing the catalyst, and an exhaust gas purification apparatus comprising the same. on metal oxide support treated by hydrothermal treatment for carbon monoxide oxidation}

The present invention relates to a catalyst for oxidation of carbon monoxide comprising a support containing metal oxide nanoparticles and noble metal nanoparticles fixed to the support, a process for producing the catalyst, and an exhaust gas purification apparatus comprising the catalyst.

Precious metal nanoparticles (NPs) such as platinum (Pt), rhodium (Rh), and palladium (Pd) supported on oxides are typical components of automotive three-way catalyst devices. Since rare and expensive metals are used in these 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, resulting in high cost efficiency. However, smaller metal nanoparticles tend to thicken during the catalytic reaction, which is due to the phenomenon that the metal nanoparticles are sintered. This is due to the high surface energy of the metal nanoparticles, which thermodynamic instability increases exponentially as the size of the metal nanoparticles decreases. Smaller nanoparticles tend to be sintered more easily, so to ensure high catalytic activity, the size of the nanoparticles should be small and not sintered. In addition, because the exhaust environment of a real car is very severe, modern automobile catalysts must remain active at high temperature (about 750 ° C) and air (humidity of about 10% H ₂ O). However, the conventional catalysts have a problem that their surface area is reduced in the actual exhaust environment as described above, and the catalytic active sites are lost.

Conventionally, various studies such as pretreatment, post-treatment, or production of a new catalyst structure have been carried out in order to prevent such inactivation. Among them, the redispersion phenomenon in which the size of the metal nanoparticles is reduced after a specific treatment is attracting attention as a promising strategy for regenerating the sintered metal nanocatalyst. Methods such as repeated oxidation-reduction, oxy-chlorination or chlorination, and iodinated methane treatment have been studied for treatment to induce redispersion.

Among the repetitive oxidation - reduction methods Nagai. Y. et. According to al., platinum nanoparticles supported on cerium oxide were redispersed in an oxidizing atmosphere. Tanabe. T. et. In this study, it was confirmed that platinum nanoparticles supported on magnesium oxide were heat - treated in air at 1073K and redispersed in case of reduction in 10% hydrogen atmosphere. Wu. T. et. al., it was confirmed that the platinum nanoparticles supported on cubic cobalt oxide were redispersed by repeated oxidation-reduction. Lira. E. et. al., reported that platinum nanoparticles supported on γ-Al ₂ O ₃ are oxidized and redispersed by NO. Jones. J. et. al. reported that platinum is redispersed in cerium oxide by atom trapping. Repeated oxidation-reduction processes are difficult to apply industrially because they require rapid iterations of complex processes.

Oxy-chlorination or post-chlorination treatment of Lenormand. F. et. al., it was confirmed that the sintered Pt / Al ₂ O ₃ was redispersed under the HCl / H ₂ O / O ₂ / N ₂ flow at 773K. Galisteo. FC et. The autocatalytic activity of Pt / Al ₂ O ₃ was recovered by oxy - chlorination and the diesel oxidation catalyst performance was recovered. Lambrou. PS et. According to al., palladium nanoparticles supported on CeO ₂ -Al ₂ O ₃ were redispersed by oxy-chlorination. However, such oxy-chlorination or post-chlorination post-treatment process is complicated and difficult to handle and treat due to toxicity of halogen gas generated by post-treatment.

In the iodinated methane treatment method, Goguet. A. et. al. And Duan. X. P. et. According to al., the iodinated methane treatment increased the degree of dispersion of the gold particles and maintained the catalytic activity. J. et. al., it was confirmed that the degree of dispersion of gold particles was increased by the Au-I interaction resulting from methane treatment with iodide. In the case of iodinated methane treatment, it is limited to gold in the metal. However, since gold has a very low thermal stability, it is difficult to use it as an actual automobile catalyst.

In order to solve the above problems, many researchers have studied a method for improving the stability of a supported metal catalyst. In recent years, cerium oxide (CeO ₂ ) as a support material for noble metal nano- . This is because strong metal-support interactions lead to a synergistic redox cycle for both the metal and cerium oxide, which is the preferred catalyst performance in a wide variety of applications. In addition, cerium oxide has a high oxygen storage capacity (OSC) and redox properties due to the cycle of Ce ^{4 +} / Ce ^{3 +} , contributing to high activity for CO oxidation. Among the various noble metals, palladium (Pd) can be evenly dispersed on cerium oxide due to strong interaction with cerium oxide, and as a result, the catalytic activity can be improved. Thus, a number of recent researchers are interested in low temperature CO oxidation using palladium supported on cerium oxide. X. Yang. et. al. reported that the redispersing effects of cube-shaped cerium oxide on platinum (Pt) nanoparticles are supported by an alternating atmosphere of oxidizing and reducing conditions. T. Wu. et. al. reported that redispersion of cerium oxide-based supported platinum in an actual vehicle exhaust environment is due to strong metal support interaction between platinum and cerium oxide. J. Lupescu. et. al. investigated the effect of processing environment on redispersion of palladium model catalysts. B. Goldsmith. et. al. reported decomposition and redispersion of 3-way catalysts rhodium, palladium and platinum induced by CO and NO during oxidation of toxic gases such as NOx and CO. However, a large number of Pd / CeO ₂ catalysts still undergo a severe deactivation process during high temperature and humid air treatment, which is the actual vehicle exhaust purification system environment.

In addition, when CO is oxidized using a catalyst, a carbonate layer is formed on the surface of the catalyst. In general, various carbonates that form a layer on the surface are considered to be toxic and result in a decrease in CO oxidation activity. Therefore, in order to improve the catalytic activity and durability, it is very important to remove the carbonate formed on the surface. In this regard, B. Schumacher. et. al. reported that the deactivation of Au / TiO ₂ is caused by the accumulation and deposition of carbonate species on the surface of the catalyst and not by the sintering of Au particles. T. Ntho. et. al. reported that FT-IR analysis indicates that the formation of bicarbonate on Au supported on titania nanotubes is a competitive reaction leading to catalyst deactivation. Although research for removing carbonates is essential to perform efficient CO oxidation, studies to investigate such an important topic have not been sufficiently performed so far, and the problems of carbonates formed during CO oxidation are still continuing.

Korean Patent No. 10-0206487

An object of the present invention is to provide a catalyst for carbon monoxide oxidation, which comprises a support including metal oxide nanoparticles and noble metal nanoparticles and has improved catalytic activity and durability, an automobile exhaust gas purification apparatus comprising the same, and a method for producing the catalyst.

DISCLOSURE OF THE INVENTION In view of the fact that metal nanoparticles can be redispersed when a catalyst containing metal nanoparticles immobilized on a support is subjected to hydrothermal treatment, a support including metal oxide nanoparticles and hydrothermal treatment including noble metal nanoparticles Treated carbon monoxide oxidation catalyst.

Accordingly, the present invention relates to a support for metal oxide nanoparticles and noble metal nanoparticles, wherein said noble metal nanoparticles are 0.3 to 1.6 nm in diameter.

The present invention also relates to an automobile exhaust gas purifying apparatus including a catalyst for oxidizing carbon monoxide according to the present invention.

The present invention also provides a process for producing a carbon monoxide oxidation catalyst, comprising: forming a support comprising metal oxide nanoparticles; fixing noble metal nanoparticles on the support to form a catalyst for oxidizing carbon monoxide; and hydrothermally treating the catalyst for carbon monoxide oxidation And a method for producing a catalyst for oxidizing carbon monoxide.

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

According to the present invention, a hydrothermal treatment catalyst for oxidation of carbon monoxide including a support comprising metal oxide nanoparticles and a catalyst including noble metal nanoparticles solves the problem of catalyst deactivation by applying hydrothermal treatment conditions substantially similar to actual vehicle exhaust environments , The effect of further improving the catalytic activity is achieved, and the present invention can contribute to designing an activated and stable new automobile catalyst.

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

FIG. 1 is a conceptual diagram showing that Pd nanoparticles are redispersed on a cerium oxide support by hydrothermal treatment of the catalyst of the present invention, and poisoning formation is reduced by hydroxyl groups, indicating high CO oxidation activity and durability.
Figure 2 is Example 1, prepared according to (a) Pd / CeO of the non-heat-treated Pd 2% by weight of _second catalyst, (b) of the heat-treated Pd 2% by weight Pd / CeO ₂ catalyst, (c) a hydrothermal treatment Pd 2% by weight of a HAADF-STEM image of a Pd / CeO ₂ catalyst. The white circle represents the palladium on the surface of the cerium oxide. (d) is an EDS mapping image of a hydrothermal treatment with Pd 2 wt% Pd / CeO ₂ catalyst. Red indicates palladium, and green indicates cerium.
Figure 3 (a) is a HAADF-STEM image of a cerium oxide, (b) is a cerium oxide, (c) is Pd / CeO of the non-heat-treated Pd 0.1% by weight of _second catalyst, (d) is not heat-treated Pd 2 wt. % of Pd / CeO ₂ is a TEM image of the 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 indicates palladium, and green indicates cerium.
5 (a) to 5 (e) are HR-TEM images of a Pd / CeO ₂ catalyst containing ₂ wt% of Pd without heat treatment. The red circles represent palladium. (f) is a palladium diameter distribution diagram obtained from an HR-TEM image.
6 (a) to 6 (e) are HR-TEM images of a Pd / CeO ₂ catalyst with 2 wt% of heat-treated Pd. The red circles represent palladium. (f) is a palladium diameter distribution diagram obtained from an HR-TEM image.
7 (a) to (e) are HR-TEM images of a Pd / CeO ₂ catalyst of 2 wt% Pd treated with hydrothermal treatment. The red circles represent palladium. (f) is a palladium diameter distribution diagram obtained from an HR-TEM image.
Figure 8 (a) Pd / CeO of the non-heat-treated Pd 2% by weight of _second catalyst, (b) Pd / CeO ₂ catalyst (T) of Pd 2% by weight of the heat-treated, (c) a hydrothermal treatment of Pd 2% by weight of Pd / CeO ₂ catalyst (HT), (d) 0.1 wt% Pd / CeO ₂ catalyst without heat treatment, (e) 0.1 wt% Pd / CeO ₂ catalyst with heat treatment P, Pd is the Pd 3d XPS spectrum of 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))
9 is an XRD result of Pd 0.1% by weight before and after thermal treatment or hydrothermal treatment, and 2 wt% Pd / CeO ₂ catalyst. The PdO peak disappeared after hydrothermally treating ₂ % by weight of the Pd / CeO2 catalyst.
10 is a graph showing the relationship between (a) another moisture content, (a) the amount of Pd / CeO ₂ catalyst (as-made) containing Pd 2 wt% of Pd and the Pd / CeO ₂ catalyst (b) other hydrothermal treatment temperatures, (c) other hydrothermal treatment time conditions and at a flow rate of 100 sccm (1% CO and 1% O ₂ under He) at 120,000 ml / hr _cat .
11 is a non-heat-treated (as-made), the heat-treated (T), a hydrothermal-treated (HT) XANES (X-ray of Pd 2% by weight of Pd / CeO ₂ catalyst, PdO, and Pd foil absorption near-edge structure ) Data.
FIG. 12 (a) shows a Pd / CeO ₂ catalyst of 2% by weight Pd / hydrothermally treated (HT) Pd / CeO ₂ catalyst as heat treated, heat treated, Pd K edge k ³ weighted Fury of PdO and Pd foil The converted EXAFS spectrum, the solid line is the experimental data, and the dotted line is the fitting result. The spectra of the PdO and Pd foils are multiplied by 1/3 and 1/4 times, respectively, to facilitate comparison. (b) is an overlaid FT-EXAFS spectrum of Pd / CeO ₂ catalyst with 2 wt% Pd as heat-treated (T) and hydrothermally treated (HT).
13 is a view showing the change of CO adsorption type after heat treatment or hydrothermal treatment of Pd / CeO ₂ catalyst of Pd 2 wt% by in-situ DRIFT measurement.
14 is a graph showing the results of oxidation of CO with the catalyst prepared according to Example 1 and Comparative Example 9. Fig.
FIG. 15 is a graph showing the relationship between the M / CeO ₂ catalyst and the hydrothermally treated (HT) M = Pd, Pt, Rh, Ru hydrothermally treated M / CeO ₂ catalyst at 120,000 ml / hr _cat at a flow rate of 100 sccm 1% O ₂ ).
FIG. 16 is a graph showing the relationship between Pd / CeO ₂ catalysts treated with hydrothermally treated (HT) Pd / CeO ₂ catalysts at 120,000 ml / hrg _cat at a flow rate of 100 sccm (1% ₂ ). ≪ / RTI >
Figure 17 is a non-heat-treated, heat-treated (T), a hydrothermal-treated (HT) 1% CO and 1% under a Pd / CeO ₂ catalyst of Pd 2% by weight of a flow rate of 100 sccm at 120,000 ml / hrg _cat (He O ₂ ). FIG. '2.0 H ₂ O' indicates the result of oxidizing CO by contacting Pd / CeO ₂ catalyst with Pd 2 wt% of untreated Pd / CeO ₂ catalyst to CO and O ₂ flow containing 3.2% H ₂ O.
FIG. 18 is a graph showing the results of CO oxidation under conditions of a low 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 1% CO ₂ flow.
19 is a graph showing in-situ DRIFT results of a Pd / CeO ₂ catalyst of hydrothermally treated Pd 2% by weight. Flowing a 2% CO and Ar 20 minutes at 25 ℃, and is made a _2% O 2, including Ar for obtaining the IR spectrum at 25 to 125 ℃ After purging the residual CO gas to flow in a vacuum for 10 minutes.
FIG. 20 is a graph showing changes in catalytic activity for CO oxidation according to the content of water contained in the CO stream using Pd / CeO ₂ catalyst of Pd 2 wt.% Without heat treatment.
FIG. 21 (a) is a graph showing various hydrothermal treatment times, and FIG. 21 (b) is a graph showing CO oxidation results repeatedly reacted with a hydrothermally treated catalyst for 25 hours.
FIG. 22 (a) is a graph showing the relationship between CO (CO) content when Pd / CeO ₂ catalyst of 2 wt% Pd which is as-made, hydrothermally treated (HT), hydrothermally treated at 570 ° C. And the rate of oxidation reaction. (b) is a graph showing the relationship between ln r and 1 / T from the reaction rate.
23 (a) is a graph showing the results of TPD of a Pd / CeO ₂ catalyst containing 2 wt% of Pd hydrothermally treated with H ₂ ¹⁸ O. FIG. H ₂ ¹⁶ O (m / z = 18) and H ₂ ¹⁸ O (m / z = 20) were monitored by mass spectrometry. (b) shows temperature-programmed reaction spectroscopy (TPRS) results of Pd / CeO ₂ catalyst of Pd 2 wt% hydrothermally treated with H ₂ ¹⁸ O. FIG. CO flow was repeated 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).
24 (a) shows the result of simultaneous oxidation of CO and C ₃ H ₆ using a Pd / CeO ₂ catalyst with 2 wt% of Pd (hydrothermally treated) and without heat treatment. (b) is a result of CO oxidation after sulfur (S) contamination of Pd / CeO ₂ catalyst with 2 wt% Pd heat treated (HT) and heat treated (T) without heat treatment.
25 (a) is a result of a long-term durability test for oxidation of CO with Pd / CeO ₂ catalyst of 2 wt% Pd heat-treated (T) and hydrothermally treated (HT) Pd without heat treatment. (b) is a graph showing the results of CO oxidation in the presence of 100 ppm of SO ₂ flow.
26 is a graph showing a result of performing the continuous CO oxidation at various temperatures using a hydrothermal treatment of the Pd 2% by weight Pd / CeO ₂ catalyst. At the end of each CO oxidation cycle, the sample was cooled to room temperature and proceeded 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) the Pd / CeO ₂ catalyst of Pd 2 wt% without heat treatment, and (d) the presence of surface hydroxyl groups of the Pd / CeO ₂ catalyst of 2 wt% Pd hydrothermally treated by in situ DRIFT analysis.
Fig. 28 (a) shows the result of in-situ DRIFT analysis of carbonate formation of heat-treated cerium oxide, (b) hydrothermally treated cerium oxide, (c) cerium oxide without heat treatment, (d) In-situ DRIFT analysis of the presence of hydroxyl groups on the surface.
Figure 29 is the non-cerium oxide and hydrothermal treatment (HT) of cerium oxide, with the non-heat-treated Pd 2% by weight Pd / CeO ₂ catalyst, and hydrothermal treatment (HT) Pd / CeO ₂ catalyst surface for Pd 2% by weight to the heat treatment The TPD of CO ₂ (m / z = 44) was measured by mass spectrometry after forming the carbonate.
30 is DRIFT data for sulfate formation of Pd / CeO2 catalyst with Pd 2% by weight of untreated Pd / CeO ₂ catalyst and hydrothermally treated (HT) Pd 2% by weight Pd / CeO ₂ catalyst. For comparison, DRIFT data of unheated cerium oxide, hydrothermally treated cerium oxide and Pd 2 wt.% Pd / Al ₂ O ₃ catalyst are shown.
31 (a) is a graph showing the TPD of oxygen (m / z = 32) observed by mass spectrometry of cerium oxide and Pd 2% by weight of Pd / CeO ₂ catalyst after surface carbonate formation .
TPD graph of the case 32 is passed through a CO / He in the carbonate form (C) of the non-heat-treated Pd 2% by weight before and after the Pd / CeO ₂ catalyst and the hydrothermal-treated (HT) Pd / CeO ₂ catalyst of Pd 2% by weight to be.

The present invention relates to a catalyst for oxidation of carbon monoxide, comprising a support comprising metal oxide nanoparticles and noble metal nanoparticles fixed to the support, wherein the diameter of the noble metal nanoparticles is 0.3 to 1.6 nm. By hydrothermal treatment of the catalyst of the present invention, the diameter of the noble metal nanoparticles may be as shown in the above range, and in this case, the specific surface area of the catalyst capable of exhibiting the catalytic activity required for carbon monoxide oxidation is obtained.

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

In the present invention, the noble metal may be selected from the group consisting of palladium, rhodium, platinum and ruthenium. The noble metal may be included in an amount of 0.1 to 5.0% by weight based on the catalyst for oxidizing carbon monoxide of the present invention. Preferably 2.0% by weight. When the content of the noble metal is lower than 0.1 wt%, the efficiency of carbon monoxide oxidation is remarkably lowered. When the content of noble metal is higher than 5.0 wt%, the economic efficiency is lowered and the oxidation efficiency is lowered compared with the noble 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 30 to 60 m ² / g. If 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, the hydrothermal treatment collectively refers to a treatment performed through heating in a wet environment, which is distinguished from a heat treatment performed through heating in a moisture-free environment. The catalyst for oxidation of carbon monoxide subjected to hydrothermal treatment according to the present invention may be one in which the noble metal nanoparticles on the catalyst support are redispersed. Here, the redispersion phenomenon of the noble metal nanoparticles due to the hydrothermal treatment increases the proportion of the ionic noble metal nanoparticles as the proportion of the metallic noble metal nanoparticles decreases, But may be, but is not limited to, redispersing by the particles. The redispersion of the noble metal nanoparticles may improve the catalytic activity of the catalyst for oxidizing carbon monoxide according to the present invention. In addition, the hydrolysis-treated catalyst for carbon monoxide oxidation according to the present invention can inhibit the formation of carbonate species in the carbon monoxide oxidation process. Here, the carbonate species is a toxic component that results in a decrease in carbon monoxide oxidizing activity. As a result, the activity of the carbon monoxide oxidizing catalyst is improved as the formation of the carbonate species is suppressed by the hydrothermal treatment . 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. In the case of hydrothermal treatment in air containing less than 600 ° C or H ₂ O of less than 5%, there arises a problem that the noble metal nanoparticles are not well redispersed in the metal oxide support, which leads to a sharp reduction in the carbon monoxide oxidation efficiency . If the hydrothermal treatment temperature exceeds 750 ° C, there arises a problem that the noble metal nanoparticles are sintered. In the case of hydrothermal treatment in air containing 10% or more of H ₂ O, water adheres to the surface of the catalyst excessively, A problem arises. Further, in the present invention, the hydrothermal treatment may be performed under the above temperature and humidity conditions for 19 to 30 hours. When the treatment time is shorter than 19 hours, the redispersion of the metal nanoparticles is less likely to occur, and when the treatment time exceeds 30 hours, the metal nanoparticles are sintered.

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

The present invention also provides a method for producing a carbon monoxide oxidizing catalyst, comprising: forming a support comprising metal oxide nanoparticles; fixing a catalyst containing noble metal nanoparticles on the support to form a catalyst for oxidizing carbon monoxide; The present invention relates to a method for producing a catalyst for oxidation of carbon monoxide.

Example  And Experimental Example

Hereinafter, preferred embodiments of the present invention will be described in order to facilitate understanding of the present invention. However, the following examples are provided only for the purpose of easier understanding of the present invention, and the present invention is not limited by the examples.

Example 1-1. Pd / CeO ₂  Catalyst synthesis

The cerium oxide support was prepared by the precipitation method. 1000 mg of Ce (NO ₃ ) ₃ 6H ₂ O (cancochemical) was dissolved in 23 mL of distilled water in a round bottom flask to prepare a 0.1 M aqueous solution. Thereafter, ammonia water (Ducksan) was added dropwise to the aqueous solution until the pH of the solution reached 8.5 while stirring (500 rpm) continuously to form cerium hydroxide. After stirring at 500 rpm for 1 hour, a pale yellow slurry was obtained and filtered, and washed several times with twice distilled water to remove anionic impurities. The obtained precipitates were dried in a convection oven at 80 DEG C for 12 hours, and the obtained solid was pulverized into a fine powder form in ceramic mortar. Thereafter, the obtained powder was fired at 500 DEG C for 5 hours at a ramping rate of 5 DEG 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 and vigorously stirred (900 rpm) to disperse the cerium oxide powder in distilled water. To prepare the H ₂ PdCl ₄ solution, 2: 1 molar HCl (tertiary) 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 an Na ₂ CO ₃ aqueous solution. Then, H ₂ PdCl ₄ (Pd 4 mg) and Na ₂ CO ₃ aqueous solution were introduced dropwise into the cerium oxide dispersion with stirring (900 rpm), and the pH of the final solution was adjusted to 9. Stirring was continued for 2 hours, then allowed to stand at room temperature for 2 hours, then the solution was filtered and washed several times with deionized water and then dried at 80 DEG C for 5 hours.

Pd / CeO ₂ catalysts having Pd contents of 0.1 wt%, 0.2 wt%, 0.5 wt%, 1 wt%, 2 wt%, 4 wt%, and 5 wt% were synthesized through the above method.

Additionally, were 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.

Examples 1-2. Rh / CeO ₂  Catalyst synthesis

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

Examples 1-3. Pt / CeO ₂  Catalyst synthesis

H ₂ PtCl _{6 6} H ₂ O solution was prepared and Pt / CeO ₂ catalyst of 2 wt% Pt was synthesized by the method of Example 1-1, except that platinum was used as a noble metal.

Examples 1-4. Ru / CeO ₂  Catalyst synthesis

H ₂ RuCl ₅ to prepare a solution of ethylamine in Example 1-1 by way of Ru / CeO ₂ catalyst Ru 2% by weight of ruthenium, except for using as the noble metal.

Comparative Example 1. Ir / CeO ₂  Catalyst synthesis

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

Comparative Example 2. Au / CeO ₂  Catalyst synthesis

A Au / CeO ₂ catalyst of 2 wt% Au was synthesized by the method of Example 1-1 except that a HAuCl ₄ 3H ₂ O solution was prepared and gold was used.

Comparative Example 3: Cu / CeO ₂  Catalyst synthesis

H ₂ CuCl ₄ solution was synthesized and prepared by the method of Example 1-1 in the Cu / Cu 2% by weight of CeO ₂ in the catalyst except for using the copper.

Comparative Example 4 Co / CeO ₂  Catalyst synthesis

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

Comparative Example 5. Ni / CeO ₂  Catalyst synthesis

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

Example 2. Pd / TiO ₂  Catalyst synthesis

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

Comparative Example 6 Pd / ZrO ₂  Catalyst synthesis

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

Comparative Example 8 Pd / Al ₂ O ₃  Catalyst synthesis

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

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

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

Experimental Example

Catalytic CO oxidation process

50 mg of the catalyst prepared according to the Examples and Comparative Examples were injected into a U-type quartz cell. At this time, the catalysts without heat treatment, the heat treated catalyst and the hydrothermally treated catalyst were compared. The reactor was purged with 100 sccm He gas at 100 < 0 > C for 1 hour and cooled to room temperature under a 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 from 25 占 폚 to 10 占 폚 at a rising rate of 5 占 폚 / min until a CO conversion of 100% was reached. When the temperature reached each temperature, the reactor was maintained for 18 minutes to achieve equilibrium.

Hereinafter, the effect according to the presence of propylene was confirmed in the same manner as above except that 100 sccm of a feed gas composed of 1% CO, 0.2% C ₃ H _6, and 10% O ₂ under He gas flow was used . In the following, the influence of sulfur pollution was confirmed by the same method as above, except that 100 ppm of SO ₂ / He flow was flowed at 100 sccm for 2 hours at 300 ° C on the catalyst. In some cases, water was flowed with CO and O _2, and the moisture content was controlled by bubbling the inlet flow of the water tank at various temperatures.

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

In-situ diffuse reflectance Fourier-transform infrared spectroscopy (DRIFT) was measured using a Nicolet iS-50 (Thermo Scientific). To do this, 13 mg of catalyst was mixed with 86 mg of KBr and pulverized, and the pulverized powder was injected into a sample cup and mounted in a DRIFT cell. The cell was covered with a KBr window and purged with Ar gas at 98 sccm for 20 minutes at room temperature. The cell was purged with Ar flow at 100 < 0 > C for 1 hour to remove water and cool to room temperature. Thereafter, the DRIFT cell was heated at 85 ° C. for 20 minutes and the temperature was maintained at 85 ° C. because the maximum difference in activity between the prepared, heat-treated, and hydrothermally treated catalysts at that temperature . Thereafter, 2% CO 2 gas was charged at 85 캜 for 10 minutes. The DRIFT spectrum was obtained in vacuum and 2% O ₂ gas was charged at 85 ° C for 10 minutes and the DRIFT spectrum was obtained simultaneously. Finally, additional spectra were obtained in vacuum.

How to identify characteristics of catalyst

Transmission electron microscope (TEM) images were obtained using a Tecnai TF30 ST device (300 kV). Also, 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 (HAADF-IR) spectroscopy using titanium-cube G2 60-300 (FEI, 300 kV acceleration voltage) STEM images were obtained. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis was performed on the OPTIMA 7300 DV instrument to confirm the metal content in the catalyst.

The crystal structure of the catalysts was observed through an X-ray diffraction (XRD) pattern (Rigaku, Cu Kα radiation). The state of the metal was confirmed by X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo VG Scientific). The bond energies were calculated using the maximum intensity of the C 1s signal, which is advantageous 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. This method utilizes a pre-injection of CO ₂ to prevent overestimation of noble metal nanoparticle dispersion caused by adsorption of CO on the support as a carbonate species. First, sikyeotgo heated to about _{50 mg 2% 3.5% O 2} / He gas in 300 ℃ (10 ℃ / min) in the catalyst having a weight for 10 minutes, then was let cooled to 50 ℃, the sample 4.9% H ₂ / Ar stream to 200 < 0 > C and cooled to 50 < 0 > C. The samples were then exposed to gas flow in the following sequence: (i) He (5 min); _{(ii) 3.5% O 2 /} He (5 minutes); (iii) CO ₂ (10 min); (iv) He (20 min); (v) 5% H ₂ / Ar (5 min). Finally, the CO was pulsed every minute in the He stream until the adsorption of CO on the sample was saturated.

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

Confirmed changes in diameter of metal due to water heat treatment

An electron micrograph of the unheated Pd / CeO ₂ catalyst, the heat-treated Pd / CeO ₂ catalyst and the hydrothermally treated Pd / CeO ₂ catalyst sample prepared according to Example 1 is shown in FIG. The approximate diameter of the cerium oxide was about 15-30 nm as shown in Fig. When 2 wt% 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, when the prepared catalyst was heat-treated at 750 ° C in wet air of 10% H ₂ O for 25 hours, the Pd domain was hardly observed. The EDS mapping image of the sample hydrothermally treated in Figure 2 (d) clearly shows that Pd is uniformly distributed on the cerium oxide, but the 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 a distinct domain as shown in FIG. The high resolution TEM images of Figures 5 to 7 show that 2 wt% Pd / CeO ₂ The catalysts showed an average diameter of 1.5 nm for the Pd domain, 2.2 nm for the heat-treated sample, and 0.9 nm for the hydrothermally treated sample. These results demonstrate that the Pd domain increases after heat treatment and the Pd domain decreases after hydrothermal treatment.

The reduction of Pd domain diameter after hydrothermal treatment, also called redispersion phenomenon, could be confirmed by the chemical adsorption of CO. BET surface area of the catalyst sample, showed a Pd dispersion and changes in diameter, Pd oxidation state and the T ₁₀₀ is estimated 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 (° C) ^d Cerium oxide 58.7 - - - 270 Heat-treated cerium oxide 4.6 - - - 300 Hydrothermally treated cerium oxide 4.9 - - - 290 The 2 wt% Pd / CeO ₂ 57.4 59.1 1.5 0.40 125 Heat-treated 2 wt% Pd / CeO ₂ 38.5 36.7 2.4 0.22 145 The hydrothermally treated 2 wt% Pd / CeO ₂ 39.9 97.8 0.9 0.14 75

a: Pd dispersion was measured by pulsed CO chemisorption.

b: The diameter of the Pd particles was estimated from the Pd dispersion.

c: estimated from the Pd 3d XPS spectrum.

d: the temperature at which the CO conversion rate reaches 100%.

As shown in Table 1, changes in the BET surface area and Pd oxidation state were measured to confirm the physicochemical properties of the Pd / CeO ₂ catalyst according to 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 ² / g, and it was confirmed that cerium oxide aggregation was inhibited by addition of Pd. After hydrothermal treatment, the Pd dispersion increased to 97.8%, and the diameter of the Pd nanoparticles decreased to 0.9 nm, confirming that the redispersion of the Pd nanoparticles occurred. The change in 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 was significantly reduced after hydrothermal treatment. Changes in the crystal structure were also observed in XRD as shown in Fig. An XRD peak for PdO was observed in the Pd / CeO ₂ catalyst of 2 wt% Pd, but after the hydrothermal treatment, the PdO peak disappeared and redispersion of the Pd domain was also confirmed.

Determination of effect of hydrothermal treatment condition on catalyst

The influence of hydrothermal treatment conditions on the Pd / CeO ₂ catalyst of 2 wt% Pd was confirmed. The Pd domain size was estimated as shown in Table 2 by varying the moisture content, temperature and treatment time. The average Pd diameter decreased as the water content increased to 10%. Also, the heat treatment at a temperature of 750 캜 for 25 hours was an optimal treatment condition for redispersion of Pd supported on cerium oxide. The CO oxidation activity of hydrothermally treated Pd / CeO ₂ under various conditions is shown in FIG. In the water content experiment, the catalytic activity increased up to 10%, but as the water content increased, excessive amount of water blocked the active sites and the catalytic activity was greatly reduced. In experiments with different temperatures and times, the activity of the further redispersed catalyst was better, indicating that the Pd redispersion strategy is effective in improving CO oxidation activity.

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

Table 3 shows the change in the size of the Pd domain with respect to the hydrothermal treatment time in order to further confirm the Pd redispersion phenomenon. As a result, it was confirmed that the Pd domain increased from 1.5 nm to 2 nm by 4 hours and then decreased gradually. Hydrothermal treatment can provide a surface-OH group 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 Pd ^{2 +} -OH species formed migrate with high mobility, and Pd supported on cerium oxide is redispersed due to strong interaction between Pd ^{2 +} -OH and cerium oxide.

Heat 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 Heat 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 Heat 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

Enhanced metal-support interactions after hydrothermal treatment were confirmed by XANES and EXAFS. The change of Pd oxidation state after hydrothermal treatment was evaluated by using XANES as shown in FIG. The XANES data showed that the white line peak of hydrothermally treated Pd / CeO ₂ migrated to higher energy than the untreated catalyst sample and the heat treated sample. The highly oxidized Pd in the hydrothermally treated sample appears to be due to the formation of Pd ²⁺ -OH. The Pd K edge EXAFS data also includes the Pd / CeO ₂ catalyst sample, the heat-treated Pd / CeO ₂ catalyst sample, the hydrothermally treated Pd / CeO ₂ catalyst sample, the PdO and Pd The results of the fitting 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 considered to enhance the interaction of Pd-O-Ce with decreasing Pd domain size.

Sample pass The coordination number (R) Atomic distance (Å) The Db-Waller factor (? ² /? ² ) R-factor (%) Not heat treated.
No samples 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 Hydrothermally treated
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

After adsorption of CO to the metal reaction site, in-situ IR analysis was performed to obtain important information on the metal structure. CO molecules linearly adsorbed to a single Pd site show peaks at 2000 to 2200 cm ^-1 while CO adsorbed to Pd ensemble sites with a bridge mode or a 3-fold hollow mode have a peak at 1900 to 2000 cm ^-1 Or a peak at 1800 to 1900 cm ^-1 . Figure 13 shows the IR peaks of the CO adsorbed on the catalyst, the catalyst heat-treated and hydrothermally treated Pd / CeO ₂ samples that are not heat treated. The heat treated samples showed large Hollow CO peaks due to Pd agglomeration. However, in the hydrothermally treated sample, the bridge or the hollow CO peak almost disappeared, and the linear CO peak remarkably increased to support the Pd redispersion phenomenon. XANES data, EXAFS data, and in-situ IR data all indicate that the Pd domains on the Pd / CeO ₂ catalyst were redispersed during hydrothermal treatment because oxidized Pd ^{2 +} -OH was formed and the metal-support interaction was enhanced.

To investigate the phenomenon of Pd redispersion, prepared Pd / CeO ₂ was heat-treated under He flow to control the initial Pd diameter to 1.5 to 17.8 nm, and Pd 2 wt% Pd / CeO ₂ catalyst of various diameters As shown in Fig. 5, the redispersion of the initial Pd domain size was observed by hydrothermal treatment. Pd nanoparticles with diameters of 1.5, 1.6, and 2.3 nm exhibited almost 100% dispersion and were fully redispersed. However, Pd nanoparticles of larger diameter were significantly reduced in Pd domain size, but not completely redispersed. Redispersion can occur repeatedly, as shown in Table 6. The redispersed Pd samples were sintered through heat treatment and re-dispersed by hydrothermal treatment. The redispersion was reversible up to 13 times, and it was confirmed that all the Pd was completely redispersed after hydrothermal treatment. Thus, the hydrothermal treatment process can be repeatedly applied to the regeneration process through redispersion of the sintered Pd metal catalyst in practical industrial applications.

Heat treatment temperature (캜) 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 at 100 sccm of He for 2 hours at various temperatures before hydrothermal treatment.

cycle Before heat treatment 1st round R 1st round S Second round R Second round S 3 times R 3rd turn S 4th order R Fourth turn S Pd diameter (nm) 1.5 0.9 2.1 1.0 2.1 0.9 2.2 0.9 2.2 cycle 5th order R 5th turn S 6th R 6 times S 7th order R Seventh round S 8th order R 8 times S 9 times R Pd diameter (nm) 1.0 2.2 0.9 2.3 0.9 2.3 0.9 2.3 1.0 cycle 9 times S 10 times R 10 times S 11 times R 11 times S 12 times R 12 times S 13 times R Pd diameter (nm) 2.3 0.9 2.2 1.0 2.3 0.9 2.3 0.9

'R' means redispersing by hydrothermal treatment at 750 ° C for 25 hours using a 10% H ₂ O flow, 'S' means sintering by thermal annealing at 500 ° C for 2 hours using He flow it means.

Confirmation of metal dispersion phenomenon by type of catalyst support

Pd redispersion supported on cerium oxide by hydrothermal treatment was confirmed by various experiments and it was confirmed whether or not Pd redispersion also occurs in other metal oxide supports as shown in Table 7. [ As a result, Pd was re-dispersed in the TiO ₂ support after hydrothermal treatment. On the other hand, Pd nanoparticles were remarkably 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-support interaction is essential to induce Pd redispersion through hydrothermal treatment.

catalyst
(Pd 2% by weight) Dispersion of untreated catalyst (%) Pd diameter (nm) of the untreated catalyst Dispersion (%) of hydrothermally treated catalyst 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 catalysts of Example 1 and Comparative Example 9 were used to confirm CO oxidation activity. A catalyst containing 2% by weight of Pd was used. In Fig. 14, " Ref lean " means that the catalyst of Comparative Example 9 was flowed with 0.1% O ₂ and 10% H ₂ O at 700 ° C under N ₂ condition. The reaction conditions were that 50 mg of each catalyst was contacted with 1% CO and 1% O ₂ , SV = 120,000 ml / g _cat · h under He flow.

In the case of an exhaust purification catalyst for an automobile, it is very important to remove all of CO at 150 ° C or less. However, in the case of using the catalyst of Comparative Example 9, it can be seen that all the CO is removed only when it exceeds 150 ° C. Jason. Take it. al., it was confirmed that Pd was redispersed even when dry lean treatment was carried out after the rich treatment using the catalyst of Comparative Example 9. In the case of using the catalyst of Comparative Example 9, PdO was important for redispersion of Pd. On the other hand, in the case of the catalyst of Example 1, Pd redispersion occurred only when hydrothermally treated. This means that Pd ²⁺ -OH plays an important role in redispersion. In the case of the present invention, the catalytic activity effect of hydrothermal treatment contributes not only to redispersion of Pd but also to formation of surface hydroxyl groups.

Confirm redispersion phenomenon according to metal type

After the various transition metals were supported on a 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 and Ni metals were sintered considerably after hydrothermal treatment. During hydrothermal processing, the metal species is converted to the M ²⁺ -OH form. In addition, many OH groups are formed on the surface of the cerium oxide. For metal redispersion, the mobility of the M ²⁺ -OH species should be better than the metal or metal oxide, and the interactions of the metal and the support should be stronger than the cerium oxide on the clean surface with OH on the surface. That is, Pd, Rh, Pt, and Ru correspond to this theory and cause metal redispersion after hydrothermal treatment.

catalyst
(Metal 2% by weight) Dispersion of untreated catalyst (%) Metal diameter (nm) of the untreated catalyst Dispersion (%) of hydrothermally treated catalyst The metal diameter (nm) of the hydrothermally treated catalyst 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 the metals (Pd, Pt, Rh, Ru) showed higher CO oxidation activity after hydrothermal treatment due to metal redispersions. The metal redispersions provide more catalytic reaction sites than the initial sample, further enhancing the CO oxidation activity. The catalytic activities for CO oxidation were in the order of Pd> Pt> Rh> Ru (Fig. 15).

Catalytic activity and durability for CO oxidation

The catalytic activity of the Pd / CeO ₂ catalyst having various Pd contents for CO oxidation was as shown in FIG. The cerium oxide showed almost no activity up to 200 DEG C, but the activity was improved by adding 0.1 wt% of Pd alone. The catalyst activity increased as the Pd content of the catalyst sample increased to 5 wt%. In the case of hydrothermally treated catalyst samples, the activity was increased up to 2% by weight and no further increase in Pd content showed a significant difference in activity. Pd nanoparticles of larger diameter (4.0 wt% to 5.0 wt%) hardly redispersed. The total redispersion was observed only up to 2 wt%. The catalytic activity for CO oxidation increased to 2 wt% Pd content. The activity and durability were further investigated using a catalyst having a Pd content of 2 wt%.

The temperature at which the CO conversion rate reached 100% (hereinafter referred to as T ₁₀₀ ) was determined by comparing the Pd / CeO ₂ catalyst prepared in Example 1 with Pd / CeO ₂ The catalyst had a temperature of 145 캜, and the Pd / CeO ₂ catalyst having a hydrothermally treated Pd 2% by weight was 75 캜 (Fig. 17). The hydrothermally treated samples exhibited improved catalytic activity despite reduced surface area due to Pd redispersion. The catalytic activity of the heat treated samples was lower due to the Pd sintering, and the reactivity was improved as the Pd nanoparticles were redispersed after hydrothermal treatment. As shown in Fig. 18, when the space velocity was slow or the composition of oxygen was increased, CO conversion reached 100% even at 40 占 폚, which corresponds to a very low temperature in the case of the hydrothermally treated sample.

In situ DRIFT analysis was performed to identify the actual active sites for CO oxidation at low temperatures. Figure 19 shows that Pd ^{2 +} site is causing the CO oxidation at low temperature. Peaks at 2074 and 2106 cm ^-1 indicate linearly adsorbed CO at the Pd ⁰ site and peaks at 2152 cm ^-1 indicate linearly adsorbed CO at the Pd ^{2 +} position. The CO-Pd ^{2 +} peak disappeared between 25 ° C and 75 ° C, and the CO-Pd ⁰ peak disappeared between 65 ° C and 125 ° C. This means that CO oxidation activity at low temperatures occurs mainly in redispersed Pd ²⁺ sites.

When water was introduced with CO and O ₂ , the activity of the Pd / CeO ₂ catalyst of 2 wt% Pd produced was improved, not as much as the hydrothermally treated catalyst sample. It has been reported that the addition of moisture to the conventional feed gas stream can significantly improve the activity of the metal catalyst. The water content in the feed stream was verified by various adjustments up to 10% as shown in Fig. When the moisture content was 3.2%, the catalytic activity was the highest and the activity decreased as the water content increased. Addition of water has an effect of promoting CO oxidation by increasing the amount of hydroxyl groups on the catalyst surface. The hydrothermally treated catalyst exhibits the best activity due to the effect of redispersion of Pd and the CO oxidation effect due to hydroxyl group formation on the catalyst surface.

The hydrothermal treatment time was checked by variously adjusting as shown in FIG. 21 (a). The catalyst samples according to Example 1 were hydrothermally treated for 12 to 50 hours each. It showed the best activity when hydrolyzed for 25 hours. The BET surface area after hydrothermal treatment was 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 redispersion of Pd and after 50 hours treatment the surface area has decreased too much. The CO oxidation was repeated 5 times for the catalyst subjected to heat treatment for 25 hours as shown in Fig. 21 (b), but no change in activity was observed.

Pd domain size is the same as the non-heat-treated Pd / CeO _2, to prepare a Pd / CeO ₂ catalyst having a hydroxyl group of more surface hydroxyl groups of the redistribution and the surface of Pd was observed independently of the effect on CO oxidation. The catalyst sample prepared according to Example 1 was heat treated at 570 ° C for 2 hours at 570 ° C and hydrothermally treated (expressed as 570-HT) at 750 ° C for 25 hours, resulting in a Pd domain size of 1.5 nm, . 22 shows the results of CO oxidation for this catalyst. The activity of this catalyst was higher than that of the untreated sample but lower than that of the hydrothermally treated sample. This means that both the Pd redispersion and the surface hydroxyl group play a role in improving the CO oxidation activity. The activation energies of the untreated catalyst, 570-HT and hydrothermally treated samples were 57.0, 53.8 and 53.5 kJ / mol, respectively. As such, surface hydroxyl groups play an important role in reducing activation energy. FIG. 23 shows whether or not the surface hydroxyl groups directly participate in carbon monoxide oxidation by isotope experiment. The hydrothermal treatment was carried out 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 the oxygen contained in the surface hydroxyl group was not directly involved in CO oxidation.

To evaluate the potential of the hydrothermally treated sample as a diesel oxidation catalyst, CO and C ₃ H ₆ were simultaneously charged into the reactor and the reaction results are shown in FIG. 24 (a). When C ₃ H ₆ was present, T ₁₀₀ increased from 75 ° C to 145 ° C. Nonetheless, it was found that CO and C ₃ H ₆ were fully oxidized at 145 and 155 ° C, respectively, and almost met the DOE target for LEV III, a low-emission automotive program.

Resistance to sulfur pollution is a very important problem in diesel oxidation catalysts because sulfur contained in automobile exhaust can pollute catalyst surfaces and degrade catalyst activity. In particular, cerium oxide is known to be well contaminated with sulfur. By weight of Pd 2% Pd / CeO ₂ catalyst prior to performing the CO oxidation reaction, was previously deliberately flowing into the SO ₂ 100 ppm of a source of contamination for 2 hours. The CO conversion at this time is shown in Fig. 24 (b). T ₁₀₀ increased from 125 ° C to 235 ° C in the untreated sample, 145 ° C to 285 ° C for the heat treated sample, and 145 ° C for the hydrothermally treated sample. For the hydrothermally treated samples, the increase in T ₁₀₀ was the smallest, and even in the presence of sulfur, all CO was completely oxidized below 150 ° C. Thus, it was confirmed that the hydrothermal treatment significantly increased the sulfur resistance of the Pd / CeO ₂ catalyst.

With CO at 150 ℃ using a Pd / CeO ₂ catalyst of Pd 2% by weight it was tested for long-term stability of the catalyst by oxidation for a long period of time. The results are shown in Fig. 25 (a). The catalytic activity of the untreated catalyst sample began to decrease for 27 hours and that of the heat treated catalyst sample after 8 hours, while the hydrotreated catalyst sample maintained the 100% CO conversion for 34 hours. The activity of the heat-treated samples was significantly reduced, and the hydrothermally treated samples showed a similar tendency until 60 hours, but the activity decreased. In the case of long-term oxidation of CO, the hydroxyl groups on the surface of the catalyst are largely eliminated. Nevertheless, the durability of hydrothermally treated samples was improved in the presence of 3.2% water. As shown in FIG. 25 (b), when 100 ppm of SO _{2 is} simultaneously flowed together with CO and O ₂ , the 100% CO conversion of the hydrothermally treated sample is maintained until 14 hours, It was confirmed that the high CO conversion rate was maintained for 60 hours. When the untreated catalyst sample was subjected to CO oxidation in the presence of SO ₂ , the catalytic activity dropped significantly from the initial stage, and the conversion was 0% after 35 hours. It was confirmed that hydrothermal treatment helps to maintain high catalytic activity even in the presence of sulfur.

In addition, the durability of the hydrothermally treated Pd 2 wt% Pd / CeO ₂ catalyst was tested according to the temperature change. As shown in Fig. 26, CO oxidation was repeatedly carried out at various temperatures ranging from 75 to 850 占 폚. Performing CO oxidation to one target temperature, cooling the reactor to room temperature and then performing again at the next target temperature. The hydrothermally treated samples exhibited high durability despite rapid temperature changes and showed almost no change in CO oxidation activity.

Surface carbonates and lattice oxygen

Carbonates are formed on the surface of the catalyst during CO oxidation, which is well known as a source of catalyst degradation. The degree of carbonate formation was confirmed using in-situ DRIFT (Fig. 27 (a) and (b)). It was confirmed that the carbonate was formed in the form of bidentate carbonate (1585 and 1299 cm ^-1 ) or monodentate carbonate (1482 and 1416 cm ^-1 ). The infrared peak intensity of the carbonate was found to decrease dramatically after hydrothermal treatment. Figures 27 (c) and (d) show that the hydrothermally treated sample contained much more hydroxyl groups than the non-heat treated sample. The hydroxyl group on the surface of the catalyst may interfere with carbonate formation on the surface. This, tendency such as Pd / CeO ₂ catalyst, even if the analysis in-situ DRIFT for ceria as illustrated in Figure 28 was observed.

The carbon dioxide formed on the catalyst surface during the CO oxidation was analyzed using CO ₂ temperature elevation desorption (TPD) (FIG. 29). CO and O ₂ were introduced together under the same conditions as the CO oxidation in Experimental Example 1 and purged with He to remove the physically adsorbed gas from the catalyst and then the CO ₂ desorbed by mass spectrometry was detected, Respectively. The untreated catalyst showed a large amount of carbonate on the surface, and a CO ₂ desorption peak was observed at 250, 410, and 550 ° C. However, the hydrothermally treated catalyst inhibited the carbonate formation and the CO ₂ peak was hardly detected. Thus, it was confirmed that the hydrothermal treatment suppressed carbonate formation on the surface.

Sulfur can form a sulfate on the catalyst surface to deactivate the supported metal catalyst on the support. Untreated or hydrothermally treated Pd / CeO ₂ catalyst was intentionally contaminated with 100 ppm SO _2, and then the surface sulfate formed was observed using DRIFT (FIG. 30). The intensity of the sulfate IR peak was found to decrease after hydrothermal treatment. The cerium oxide not supported with metal or the cerium oxide treated with hydrothermal treatment was also contaminated with 100 ppm SO ₂ , and the IR peak was confirmed. As a result, it was confirmed that the strength of the sulfate peak was considerably reduced after 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 the sulfate is mainly formed on the surface of cerium oxide, and the formation of sulfate on the surface of cerium oxide is considerably inhibited by hydrothermal treatment.

Hydroxyl groups and carbonates on the surface can affect the transport of oxygen. 31 shows O ₂ -TPD results in the presence or absence of carbonate on the surface in a Pd / CeO ₂ catalyst with cerium oxide and Pd 2 wt% Pd. All samples were pretreated at 300 ° C for 1 hour under an O ₂ flow. 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 and a bulk lattice oxygen desorption peak were observed at 260 ° C. and 530 ° C., respectively, at a relatively low temperature according to hydrothermal treatment to produce hydroxyl groups on the catalyst surface. On the other hand, the untreated catalyst sample showed a desorption peak of surface lattice oxygen at 300 ° C and a bulk lattice oxygen at 550 ° C. The surface carbonates were formed by flowing CO and O ₂ at room temperature and O ₂ -TPD was performed. The untreated catalyst samples showed no O ₂ desorption peaks, but hydrothermally treated catalyst samples still showed large O ₂ peaks for surface lattice oxygen at 270 ° C and bulk lattice oxygen at 520 ° C. Thus, the transfer of oxygen was interrupted by the carbonate on the surface in the untreated catalyst sample, but hydrothermally treated catalyst samples were substantially unaffected by inhibition of carbonate formation.

The TPD analysis was performed on the CO / He flow and is shown in FIG. The hydrothermally treated Pd / CeO ₂ showed a large peak at 83 ° C regardless of carbonate formation. However, untreated catalyst samples showed CO ₂ peaks at 129 ˚ C only in the uncontaminated state, and CO ₂ peaks were almost absent in the carbonate-formed samples. The amount of desorbed CO ₂ was estimated as shown in Table 9. The hydrothermally treated catalyst produced a large amount of carbon dioxide, but the amount of carbon dioxide after the carbonate formation in the untreated catalyst sample was remarkably decreased. Thus, hydrothermal treatment promotes lattice oxygen migration and interferes with carbonate formation, thus confirming high activity and durability against CO oxidation.

Catalyst (Pd 2% by weight) ^b Main CO ₂ peak temperature (캜) 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 Confirm conditions: catalyst: 50 mg, pretreatment: 100 sccm He at 100 ° C for 1 hour, TPD analysis: balance with 10% CO and He at 50 sccm, elevated rate of 10 ° C / min.

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

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

The present invention has been described by way of examples. It will be understood by those skilled in the art that the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. It is therefore to be understood that the above-described embodiments are to be considered in all respects as illustrative and not restrictive. The scope of the present invention is defined by the appended claims rather than the detailed description and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be construed as being included within the scope of the present invention.

Claims (12)

A support comprising metal oxide nanoparticles; And
And noble metal nanoparticles fixed to the support,
Wherein the noble metal nanoparticles have a diameter of 0.3 to 1.6 nm. The method according to claim 1,
And the metal oxide is cerium oxide or titanium oxide. The method according to claim 1,
Wherein the noble metal is at least one selected from the group consisting of palladium, rhodium, platinum and ruthenium. The method according to claim 1,
And 0.1 to 5.0% by weight of a noble metal. The method according to claim 1,
Wherein the specific surface area of the catalyst is 30 to 60 m ² / g. An automobile exhaust gas purifying apparatus comprising the catalyst for oxidizing carbon monoxide according to any one of claims 1 to 5. Forming a support comprising metal oxide nanoparticles;
Fixing noble metal nanoparticles on the support to form a catalyst for oxidizing carbon monoxide; And
And hydrothermally treating the carbon monoxide oxidation catalyst.
A method for producing a catalyst for oxidizing carbon monoxide. 8. The method of claim 7,
Wherein the metal oxide is cerium oxide or titanium oxide. 8. The method of claim 7,
Wherein the noble metal is at least one selected from the group consisting of palladium, rhodium, platinum and ruthenium. 8. The method of claim 7,
Wherein the hydrothermal treatment is carried out at a temperature of 600 to 750 占 폚. 8. The method of claim 7,
Wherein the hydrothermal treatment is carried out in air containing 5 to 10% H ₂ O. The method according to claim 10 or 11,
Wherein the hydrothermal treatment is carried out for 19 to 30 hours.

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