KR102368521B1 - manufacturing method of composite metal oxide catalyst for hydrogen generation using liquid phase plasma reaction and composite metal oxide catalyst - Google Patents

Abstract

The present invention relates to a method for manufacturing a composite metal oxide catalyst for hydrogen generation using a liquid phase plasma reaction and a composite metal oxide catalyst, and to a method for manufacturing a composite metal oxide catalyst capable of increasing hydrogen production efficiency and to a composite metal oxide catalyst.

Description

Manufacturing method of composite metal oxide catalyst for hydrogen generation using liquid phase plasma reaction and composite metal oxide catalyst

The present invention relates to a method for producing a composite metal oxide catalyst for hydrogen generation using a liquid phase plasma reaction and a composite metal oxide catalyst, and to a method for producing a composite metal oxide catalyst capable of increasing hydrogen production efficiency and to a composite metal oxide catalyst.

Hydrogen energy is attracting attention as a future energy to replace fossil fuels with clean energy. As climate change becomes more serious, measures to reduce the generation of carbon dioxide are being sought. For this, it is absolutely necessary to expand the use of hydrogen energy. In order to solve future energy problems, it is very important to secure more efficient hydrogen energy production technology.

Until now, hydrogen has been mostly produced in petrochemical processes based on crude oil or through steam reforming of propane or methane gas. However, this method uses hydrocarbons as starting materials and emits greenhouse gases such as carbon dioxide from the production process.

For this reason, interest in research on hydrogen production processes that do not emit carbon dioxide has recently been attracting attention. Among them, hydrogen production by water electrolysis or photolysis technology can be evaluated as a method of producing clean hydrogen when electrical energy used in the reaction is obtained from renewable energy. However, this method has a limitation in that the hydrogen productivity is low. Therefore, a more environmentally friendly and mass-produced method of hydrogen has become the focus of major research.

Plasma is generated by a high voltage discharge in various stages. Plasma discharge has the ability to initiate various chemical reactions. Plasma is typically a plasma generated in a gas phase. It is commonly applied to create fine crystals for thin films and semiconductors.

On the other hand, a technique for generating plasma in a liquid phase and using it is not yet known much. Liquid plasma is created by a high voltage that is emitted directly into the liquid. Plasma generated in water by high voltage discharge emits strong and dense ultraviolet and visible light to generate many active species. Liquid plasma simultaneously induces an electric field, strong ultraviolet rays, high-pressure shock waves, various free radical generation and ozone generation effects. This effect can destroy harmful chemicals and eliminate microorganisms in liquids.

Recently, as disclosed in Korean Patent No. 10-1814128, a technique for generating hydrogen by decomposing water by generating a liquid plasma in water in which a photocatalyst is present is known.

The patented technology uses titanium dioxide (TiO ₂ ), a type of metal oxide, as a photocatalyst.

Titanium dioxide photocatalyst exhibits excellent photoactivity in ultraviolet light, but has a problem in that photocatalytic activity is very low in visible light.

Republic of Korea Patent No. 10-1814128: Hydrogen production method using liquid plasma and photocatalyst

The present invention was created to improve the above problems, and the purpose of the present invention is to provide a method for producing a composite metal oxide catalyst for hydrogen production and a composite metal oxide catalyst, which can be photoactivated even in visible light to increase hydrogen production efficiency there is.

A method for producing a composite metal oxide catalyst for hydrogen generation using a liquid phase plasma reaction of the present invention for achieving the above object is a praseodymium (Pr) source, a strontium (Sr) source, a zinc (Zn) source, and a titanium (Ti) ) a first step of synthesizing a precursor solution containing the source; a second step of drying the precursor solution to obtain a solid product; a third step of calcining the solid product to produce a Pr-Sr-Zn-Ti-based composite metal oxide; and a fourth step of pulverizing the composite metal oxide.

The first step is a) adding praseodymium nitrate (Pr(NO ₃ ) ₃ ), strontium nitrate (Sr(NO ₃ ) ₂ ), and an organic acid to distilled water and stirring to obtain a first solution, b) the first step Zinc nitrate (Zn(NO ₃ ) ₂ ) was added to solution 1 and stirred to obtain a second solution, c) ethanolamine was added to ethanol and stirred, and then titanium alkoxide was added and stirred. obtaining a third solution; d) adding an organic acid to ethanol and stirring to obtain a fourth solution; e) adding the third and fourth solutions to the second solution and then reacting with stirring and generating the precursor solution.

The organic acid is citric acid.

In the second step, the precursor solution is dried at 110 to 150° C. for 10 to 30 hours.

The third step is to raise the temperature of the solid product at a rate of 2 to 8 ° C./min and maintain it at 250 to 350 ° C. for 1 to 3 hours, and then increase the temperature at a rate of 2 to 8 ° C. / min to 2 to 8 at 800 to 1200 ° C. Firing by holding for a period of time.

And the composite metal oxide catalyst for hydrogen generation using the liquid phase plasma reaction of the present invention for achieving the above object is Pr-Sr- to which praseodymium (Pr), strontium (Sr), zinc (Zn), and titanium (Ti) are combined. As a Zn-Ti-based composite metal oxide catalyst, it is activated by plasma generated in water to decompose water into hydrogen and oxygen.

As described above, the present invention provides a method for preparing a Pr-Sr-Zn-Ti-based composite metal oxide catalyst in which various types of metals are bonded.

Since the composite metal oxide catalyst of the present invention can be photoactivated even in visible light, it is possible to increase the hydrogen production efficiency compared to a commercially used titanium dioxide (TiO ₂ ) catalyst.

Therefore, it is possible to overcome the problem of the titanium dioxide (TiO ₂ ) catalyst, which is photoactivated only in the ultraviolet region, thereby reducing the generation of greenhouse gases and securing more efficient hydrogen energy production technology.

1 is a configuration diagram schematically showing a liquid phase plasma reactor for generating hydrogen using the composite metal oxide catalyst of the present invention;
2 is a graph showing the X-ray diffraction (XRD) pattern of the composite metal oxide catalyst,
3 is a scanning electron microscope (SEM) image of the composite metal oxide catalyst,
4 is a graph showing the energy dispersive X-ray spectroscopy (EDX) results,
5 is a graph showing the nitrogen adsorption isotherm of the composite metal oxide catalyst;
6 is a graph showing the Fourier transform infrared (FT-IR) spectrum results of the composite metal oxide catalyst,
7 is a graph showing the results of ultraviolet-visible diffuse reflectance (DRS) analysis of the composite metal oxide catalyst,
8 is a graph showing the hydrogen production rate according to the conditions for adding a composite metal oxide catalyst, a condition for adding a TiO ₂ catalyst, and a condition for not adding a catalyst.

Hereinafter, a composite metal oxide catalyst and a composite metal oxide catalyst for hydrogen generation using a liquid phase plasma reaction according to a preferred embodiment of the present invention will be described.

A method for producing a composite metal oxide catalyst for hydrogen generation using a liquid phase plasma reaction according to an embodiment of the present invention includes a first step of synthesizing a precursor solution, a second step of drying the precursor solution to obtain a solid product, and a solid product A third step of calcining to produce a Pr-Sr-Zn-Ti-based composite metal oxide, and a fourth step of pulverizing the composite metal oxide. Let's look at each step in detail.

1. Step 1

In the first step, a precursor solution is synthesized. The precursor solution contains a praseodymium (Pr) source, a strontium (Sr) source, a zinc (Zn) source, and a titanium (Ti) source.

Nitrate may be used as a source of praseodymium (Pr). For example, the nitrate may include praseodymium nitrate (Pr(NO ₃ ) ₃ ).

Nitrate may be used as a strontium (Sr) source. For example, the nitrate may include strontium nitrate (Sr(NO ₃ ) ₂ ).

And nitrate may be used as a zinc (Zn) source. For example, zinc nitrate (Zn(NO ₃ ) ₂ ) may be mentioned as a nitrate.

And as a titanium (Ti) source, it is possible to use titanium alkoxides (titanium alkoxides). For example, as titanium alkoxide, titanium tetrapropoxide, titanium tetraisopropoxide, titanium tetradiisopropoxide, titanium tetrabutoxide, titanium tetrae One selected from the group consisting of titanium tetraethooxide and titanium tetramethoopoxide may be used.

Specifically, in order to synthesize the precursor solution, a) praseodymium nitrate (Pr(NO ₃ ) ₃ ), strontium nitrate (Sr(NO ₃ ) ₂ ), and an organic acid were added to distilled water and stirred to obtain a first solution, b; ) adding zinc nitrate (Zn(NO ₃ ) ₂ ) to the first solution and stirring to obtain a second solution, c) adding ethanolamine to ethanol and stirring, and then adding titanium alkoxide Stirring to obtain a third solution; d) adding an organic acid to ethanol and stirring to obtain a fourth solution; e) adding the third and fourth solutions to the second solution and then reacting with stirring and generating a precursor solution.

First, praseodymium nitrate (Pr(NO ₃ ) ₃ ), strontium nitrate (Sr(NO ₃ ) ₂ ), and organic acid are added to distilled water and stirred to obtain a first solution.

Praseodymium nitrate (Pr(NO ₃ ) ₃ ) is used as a source of praseodymium (Pr), and strontium nitrate (Sr(NO ₃ ) ₂ ) is used as a source of strontium (Sr). Praseodymium nitrate may be used in the form of a hydrate, that is, in the form of praseodymium nitrate hexahydrate (Pr(NO ₃ ) ₃ .6H ₂ O).

The organic acid enhances the dissolution and dispersion effect of praseodymium nitrate and strontium nitrate.

As organic acids, malic acid, citric acid, aspartic acid, formic acid, acetic acid, tartaric acid, malic maleic acid, propionic acid acid), butyric acid, and valeric acid.

Preferably, citric acid is used as the organic acid. Citric acid is less harmful than other organic acids and has an excellent effect of increasing the solubility and dispersibility of praseodymium nitrate and strontium nitrate.

Next, zinc nitrate (Zn(NO ₃ ) ₂ ) is added to the first solution and stirred to obtain a second solution.

Zinc nitrate (Zn(NO ₃ ) ₂ ) is used as a zinc (Zn) source. Zinc nitrate may be used in the form of a hydrate, that is, in the form of zinc nitrate hexahydrate (Zn(NO ₃ ) ₂ ·6H ₂ O).

Next, ethanolamine is added to ethanol and stirred, then titanium alkoxides are added and stirred to obtain a third solution.

As ethanol, 2-methoxy ethanol (2-metoxy ethanol) may be used. And as the ethanolamine, at least one selected from monoethanolamine, diethanolamine, and triethanolamine may be used. Since titanium alkoxide has a strong crystallization property when it comes into contact with air, it is possible to prevent the titanium alkoxide from precipitating into crystals before the reaction by using ethanol and ethanolamine.

Titanium alkoxides are a source of titanium (Ti).

Next, an organic acid is added to ethanol and stirred to obtain a fourth solution.

As ethanol, 2-methoxy ethanol (2-metoxy ethanol) may be used. And as the ethanolamine, at least one selected from monoethanolamine, diethanolamine, and triethanolamine may be used. The fourth solution serves to increase the stability of the precursor solution.

Next, the third solution and the fourth solution are added to the second solution and then reacted to form a precursor solution. The third solution and the fourth solution may be added in the same volume ratio as the second solution. For example, the second solution: the third solution: the fourth solution may be 1:1:1 in volume ratio.

After slowly adding the third solution and the fourth solution to the second solution, the mixture is reacted with stirring for 3 to 7 hours while maintaining the temperature at 70 to 90° C. to obtain a precursor solution.

2. Step 2

The prepared precursor solution is dried to obtain a solid product.

To this end, the precursor solution may be dried at 110 to 150° C. for 10 to 30 hours.

When the precursor solution is dried, the solvent is removed, leaving a solid product.

3. Step 3

The solid product is calcined to produce a Pr-Sr-Zn-Ti-based composite metal oxide.

To this end, the solid product is heated at a rate of 2 to 8 ° C./min, maintained at 250 to 350 ° C. for 1 to 3 hours, calcined first, and then heated at a rate of 2 to 8 ° C./min to 2 to 8 at 800 to 1200 ° C. It can be held for a period of time and fired a second time.

at a low temperature of 250 to 350 °C Through primary calcination, organic matter and intermediate by-products can be burned and removed. And after the primary firing, it is finally converted into a composite metal oxide through secondary firing at a high temperature of 800 to 1200°C.

The resulting composite metal oxide is an oxide in which several types of metals are combined. For example, it is a Pr-Sr-Zn-Ti-based composite metal oxide in which four metals of Pr, Sr, Zn, and Ti are combined. Such a composite metal oxide may be represented by the following chemical formula.

Pr _a Sr _b Zn _c Ti _d O _e

In the above formula, a is 0.1 to 0.5, b is 0.5 to 1, c is 0.5 to 1, d is 0.5 to 1, and e may be 1.5 to 3.

4. Step 4

The resulting composite metal oxide is pulverized. For example, it can be pulverized to a size of 0.01 to 100 μm. Composite metal oxide in the form of pulverized fine particles is a catalyst.

The composite metal oxide catalyst of the present invention prepared as described above is a Pr-Sr-Zn-Ti-based compound in which praseodymium (Pr), strontium (Sr), zinc (Zn), and titanium (Ti) are combined.

Since the composite metal oxide catalyst of the present invention is activated not only in ultraviolet light but also in the visible light region to photolyse water, it is possible to increase hydrogen production efficiency through liquid phase plasma reaction. A liquid phase plasma (LPP) reaction that generates a high-energy plasma in a liquid can generate light energy in a liquid with various active species.

An example of a liquid phase plasma reactor for generating hydrogen using the composite metal oxide catalyst of the present invention is shown in FIG. 1 .

Referring to FIG. 1 , the liquid-phase plasma reactor includes a cylindrical reactor 10 , a cooling tank 20 for circulating water in the reactor 10 to maintain a constant temperature, and a pair of electrodes installed in the reactor 10 . 30 and a power supply (bipolar pulse power supply) 35 for supplying power to the electrode 30 .

The electrode 30 is made of a tungsten material, and the outside of the electrode 30 is covered with an insulator made of a ceramic material. The distance between the two electrodes 30 may be maintained at about 0.2 to 0.5 mm.

When power is supplied to the electrode 30 through the power supply 35, plasma is generated in the liquid by electric discharge. In order to prevent a rise in the temperature of water when plasma is generated by electric discharge, it is preferable to circulate water to the cooling tank 20 using a circulation pump to maintain the temperature of water at 18 to 25°C. The reactor 10 and the cooling tank 20 are connected by circulation lines 11 and 13 .

When power is supplied, it is preferable to supply power as a pulse (pulse width 3 to 5 μs) rather than continuously supplying power to the electrodes. When power is supplied as a pulse, it is possible to reduce the dissolution of electrode components by inhibiting the melting of the electrode exposed to the suspension.

Conditions of power supplied to the electrode to generate plasma may be a voltage of 230 to 250V, a pulse width of 3 to 5 ㎲, and a frequency of 25 to 30KHz.

And in the liquid-phase plasma reactor shown in FIG. 1, a flow controller (MFC) 45 for introducing nitrogen gas stored in the nitrogen tank 40 into the reactor in order to analyze the gas product generated inside the reactor 10, and the reactor A gas chromatograph (GC) 50 for analyzing the gas flowing out from 10 is installed.

The composite metal oxide catalyst of the present invention is added to the water accommodated in the reactor. For example, 0.001 to 1 part by weight of the composite metal oxide catalyst is added with respect to 100 parts by weight of water and stirred to uniformly disperse the photocatalyst in water.

When plasma is generated in water to which a complex metal oxide catalyst is added, various active species (H·, OH·, O·, H ₂ O ₂ , O ₂ ^- , O ₃ , etc.) are generated, and these active species generate hydrogen promotes In addition, when plasma is generated, strong ultraviolet and visible light are emitted to activate the composite metal oxide catalyst, thereby decomposing water molecules into hydrogen and oxygen by photolysis of the composite metal oxide catalyst.

Hereinafter, the present invention will be described with reference to an experimental example. However, the following experimental examples are for explaining the present invention in detail, and the scope of the present invention is not limited to the following experimental examples.

(Example: Preparation of composite metal oxide catalyst)

In 200 mL of distilled water, 0.5 mol of praseodymium nitrate hexahydrate (Pr(NO ₃ ) ₃ .6H ₂ O), 0.5 mol of strontium nitrate (Sr(NO ₃ ) ₂ ), and 0.03 mol of citric acid were injected, followed by stirring for 2 hours to obtain the first solution was obtained. Then, 0.5 mol of zinc nitrate hexahydrate (Zn(NO ₃ ) ₂ ·6H ₂ O) was injected into the first solution and stirred for 2 hours to obtain a second solution.

After injecting 0.02 mol of diethanolamine into 200 mL of 2-methoxy ethanol, stirring for 1 hour, then injecting 0.5 mol of titanium tetrabutoxide and stirring for 5 hours A third solution was obtained. Then, 0.01 mol of citric acid was injected into 100 mL of 2-methoxyethanol (2-metoxy ethanol) and stirred for 1 hour to obtain a fourth solution.

A precursor solution was synthesized by slowly injecting the third solution and the fourth solution into the second solution, and then reacting it with stirring at 80° C. for 5 hours. When synthesizing the precursor solution, the 3rd and 4th solutions were used in the same volume as the 2nd solution. The precursor solution was put into an oven and dried at 130° C. for 15 hours to obtain a solid product. Then, the solid product was put into the electric furnace, and then heated at a rate of 5 ° C./min, maintained at 300 ° C. for 2 hours, and calcined first. Then, the temperature was raised at the same rate and maintained at 600 to 1100 ° C. for 5 hours, followed by secondary calcination. . After the sintering was completed, the mixture was cooled slowly to room temperature, and then the solid product was pulverized to prepare a Pr-Sr-Zn-Ti-based composite metal oxide catalyst in powder form.

<Experimental Example 1: X-ray diffraction analysis>

The X-ray diffraction (XRD) pattern of the composite metal oxide catalyst prepared at different secondary firing temperatures (600 to 1100° C.) is shown in FIG. 2 .

Referring to FIG. 2 , the characteristic peaks of the composite metal oxide catalyst were shown at 32.35°, 39.91°, 46.42°, 57.72°, 67.74° and 77.09°. The intensity of the characteristic peaks depended on the secondary firing temperature. The degree of crystallinity, defined as the intensity of the characteristic peak, was higher for calcinations at 900, 1000, and 1100 °C than at 600, 700 and 800 °C. In particular, the crystallinity was the highest when calcined at 1100°C.

In the following experiments, a composite metal oxide catalyst prepared by calcining at 1100° C. was used.

<Experimental Example 2: Scanning electron microscope (SEM) image and energy dispersive X-ray spectroscopy (EDX) analysis>

A scanning electron microscope (SEM) image of the composite metal oxide catalyst is shown in FIG. 3, and energy dispersive X-ray spectroscopy (EDX) results are shown in FIG. 4 .

3 and 4 , it was confirmed that the micron-sized composite metal oxide catalyst was formed of cubic crystals. And the crystal size of the composite metal oxide catalyst was distributed in the range of 200 ~ 300nm. In the EDX spectrum, metal elements such as Sr, Zn, Pr, and Ti were observed along with an oxygen (O) peak. Therefore, it can be seen that the composite metal oxide catalyst is composed of a Pr-Sr-Zn-Ti-based composite metal oxide.

<Experimental Example 3: Nitrogen adsorption isotherm analysis>

5 shows the nitrogen adsorption isotherm of the composite metal oxide catalyst. 5, 'ADS' is an adsorption graph, and 'DES' is a desorption graph.

Referring to FIG. 5 , a small hysteresis curve was observed in the adsorption-desorption curve, but this appears to be due to the voids between the small crystals. And the specific surface area determined by the BET equation was as small as 0.6 m ² /g. This suggests that the composite metal oxide catalyst does not have pores. Since the optical properties of the composite metal oxide catalyst for generating hydrogen by liquid-phase plasma reaction are important, even if the specific surface area is small, the function of the catalyst is not significantly affected.

<Experimental Example 4: Fourier transform infrared (FT-IR) spectrum analysis>

6 shows the Fourier transform infrared (FT-IR) spectrum results of the composite metal oxide catalyst.

Referring to FIG. 6 , the absorption characteristic peak at 3435 cm ⁻¹ in the FT-IR spectrum is represented by OH stretching vibration. The IR spectra of the carboxylate group CO ₃ ^2- show characteristic doublet absorptions derived from symmetric and asymmetric stretching vibrations at 877, 1070 and 1456 cm ^-1 , respectively. In the IR spectrum, bands appeared by OH stretching at 3435 cm ^-1 , bands at 1456 and 1636 cm ^-1 by carboxylate group extension mode, and 877, 571 and 495 by Ti-O vibrational mode A band appeared at cm ^-1 .

In this spectrum, bands related to OH or carboxylate groups appear by the synthesis of complex compounds. In particular, the band generated by Ti-O bonding was very large. This suggests that the composite metal oxide is a composite of metal oxides in which Ti-O bonds are predominant.

<Experimental Example 5: UV-Visible Light Diffuse Reflectance Spectroscopy (DRS) Analysis>

7 shows the results of diffuse reflectance spectroscopy analysis of UV-visible absorbance of commercial TiO ₂ (P25, Degussa) and composite metal oxide catalysts expressed in Kubelka-Munk units.

Referring to FIG. 7 , in the DRS (Diffuse Reflectance UV-visible spectrophotometer) result, the absorption edge of TiO ₂ was about 380 nm, and almost no light was absorbed in the visible region. On the other hand, the DRS absorption spectrum of the composite metal oxide catalyst was observed in the upper range of visible light, indicating that the absorption range was extended. The absorption edge of the composite metal oxide catalyst was 413 nm, and the band gap was about 2.8 eV. Therefore, the composite metal oxide catalyst can be photoactivated not only by ultraviolet light but also by visible light.

<Experimental Example 6: Hydrogen Generation Experiment>

A hydrogen production experiment was performed using the liquid-phase plasma reactor of FIG. 1 .

A suspension dispersed by adding 0.3 g of a composite metal oxide catalyst to 200 mL of distilled water was injected into the reactor, and then discharged for 60 minutes under the conditions of a voltage of 240 V, a frequency of 25 kHz, and a pulse width of 6 μs to generate plasma.

In addition, experiments were carried out under conditions in which no catalyst was added and under conditions in which TiO ₂ (P25, Degussa) catalyst was added.

8 shows the hydrogen production rate according to the conditions in which the composite metal oxide catalyst was added, the conditions in which the TiO ₂ catalyst was added, and the conditions in which the catalyst was not added.

Referring to FIG. 8 , it was confirmed that a small amount of hydrogen was generated by the liquid phase plasma reaction even if a catalyst was not added. Even without a catalyst, plasma generates various active species (H·, OH·, O·, H ₂ O ₂ , O ₂ ⁻ , O ₃ , etc.) in water, which causes hydrogen production. In addition, the strong energy of plasma appears to be the result of direct decomposition of water.

When a commercially used TiO ₂ catalyst was added, it was found that the hydrogen production rate was increased. And when the composite metal oxide catalyst was added, the hydrogen production rate was improved compared to the case where the TiO ₂ catalyst was added.

Meanwhile, in the graphs of FIGS. 7 and 8 , 'composite catalyst' means 'composite metal oxide catalyst'.

As mentioned above, although the present invention has been described with reference to one embodiment, it will be understood that this is only exemplary, and that those skilled in the art may make various modifications and equivalent embodiments therefrom. Accordingly, the true protection scope of the present invention should be defined only by the appended claims.

10: reactor 20: cooling bath
30: electrode 35: power supply

Claims (6)

A first step of synthesizing a precursor solution containing a praseodymium (Pr) source, a strontium (Sr) source, a zinc (Zn) source, and a titanium (Ti) source;
a second step of drying the precursor solution to obtain a solid product;
a third step of calcining the solid product to produce a Pr-Sr-Zn-Ti-based composite metal oxide;
Including; a fourth step of pulverizing the composite metal oxide;
The first step is a) adding praseodymium nitrate (Pr(NO ₃ ) ₃ ), strontium nitrate (Sr(NO ₃ ) ₂ ), and an organic acid to distilled water and stirring to obtain a first solution, b) the first step Zinc nitrate (Zn(NO ₃ ) ₂ ) was added to solution 1 and stirred to obtain a second solution, c) ethanolamine was added to ethanol and stirred, and then titanium alkoxide was added and stirred. obtaining a third solution; d) adding an organic acid to ethanol and stirring to obtain a fourth solution; e) adding the third and fourth solutions to the second solution and then reacting with stirring generating the precursor solution;
The third step is to raise the temperature of the solid product at a rate of 2 to 8 ° C./min and maintain it at 250 to 350 ° C. for 1 to 3 hours, and then increase the temperature at a rate of 2 to 8 ° C. / min to 2 to 8 at 800 to 1200 ° C. Firing by holding for a period of time,
The composite metal oxide is represented by the following chemical formula,
Pr _a Sr _b Zn _c Ti _d O _e
In the above formula, a is 0.1 to 0.5, b is 0.5 to 1, c is 0.5 to 1, d is 0.5 to 1, and e is 1.5 to 3, A method for preparing a metal oxide catalyst. delete The method of claim 1, wherein the organic acid is citric acid. [2] The method of claim 1, wherein in the second step, the precursor solution is dried at 110 to 150° C. for 10 to 30 hours.

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