JP2010238415A - Catalyst paste preparation method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a preparation method of a catalyst paste by which a catalyst layer capable of operation under low humidification and high temperature and hardly deteriorated can be formed at a low cost, and which is superior in stability. <P>SOLUTION: The catalyst paste in which zirconium phosphate gel suspension is uniformly dispersed is obtained by stirring Pt/C catalyst dispersion liquid and dropping zirconium phosphate gel suspension into it, and the catalyst layers 14, 16 are formed using such catalyst paste, thereby, a structure in which zirconium phosphate is uniformly dispersed is obtained. As a result, high battery characteristics are obtained without using a polymer electrolyte. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for preparing an electrode catalyst paste for forming a catalyst layer that also serves as an electrode for constituting a solid polymer fuel cell.

A fuel cell uses a reducing agent such as hydrogen, methanol, or reformed hydrogen from fossil fuels as a fuel, and electrochemically oxidizes the fuel in the cell using air or oxygen as an oxidant, thereby chemical energy of the fuel. Is directly converted into electrical energy and extracted. As a result, it has higher efficiency and quietness compared to internal combustion engines, and has low emissions of NO _x , SO _x , particulate matter (PM), etc. that cause air pollution. It is attracting attention as an energy supply source. For example, it is expected to be used as a distributed power source or a thermoelectric supply system for automobile engines, residential use, etc.

  Such fuel cells are classified into alkali type, phosphoric acid type, molten carbonate type, solid oxide type, solid polymer type, and the like depending on the type of electrolyte used. Among these, the phosphoric acid form and the solid polymer form using a proton-conducting electrolyte can be operated with high efficiency without being restricted by the Carnot cycle in thermodynamics, and the theoretical efficiency is 25 (° C). It reaches 83 (%). In particular, solid polymer fuel cells have been remarkably improved in performance in recent years due to the development of electrolyte membranes and catalyst technology, and are attracting attention as a low-pollution automobile power source and high-efficiency power generation method.

The polymer electrolyte fuel cell has a structure in which gas diffusion electrodes are provided on both sides of a polymer electrolyte layer such as a plate or a cylinder via a pair of catalyst layers. Membrane Electrode Assembly (hereinafter referred to as MEA) is used in a stack structure in which separators are stacked. When the fuel gas and air are supplied to the MEA, they are diffused through the gas diffusion electrode and guided to the catalyst layer and the electrolyte layer surfaces. In this process, an oxidation reaction occurs on the catalyst layer on the fuel electrode side, protons (H ⁺ ) are generated, and flow through the electrolyte layer toward the catalyst layer on the air electrode side. Therefore, there are many reaction active points in the catalyst layer, and the generated proton and electron paths are sufficiently formed, and the diffusion of fuel gas and air guided to the catalyst layer surface through the gas diffusion electrode. It is required not to disturb.

  The catalyst layer is prepared by, for example, preparing a catalyst paste by dispersing carbon particles or the like carrying a catalyst in an electrolyte solution in which a polymer electrolyte is dissolved in an organic solvent, and applying this to the electrode to perform a drying treatment. It was formed with. As the polymer electrolyte, for example, one having high proton conductivity such as perfluorocarbon sulfonic acid (PFS) is used.

  However, the polymer electrolyte has a problem that it degrades due to decomposition by OH radicals during long-time operation. In addition, in order to ensure high proton conductivity, sufficient humidification is required, which complicates the system and contributes to an increase in cost. Therefore, operation under low or no humidification conditions is desired. In addition, from the viewpoint of improving power generation efficiency, effective use of exhaust heat, and reduction of CO poisoning of platinum catalyst, medium temperature operation of 100 (° C) or more is required. There is a problem that the operating temperature is limited to 80 (° C.) or less. Furthermore, since the resin material is contained in the catalyst paste for forming the catalyst layer as described above, there is a problem that the paste is not stable.

JP 2004-185980 A JP 2005-011697 A JP 2006-147478 A JP 2008-243688 A JP 2004-152635 A JP 2006-059634 A JP 2007-026709 A

  On the other hand, for example, it has been proposed to use a metal phosphate compound such as zirconium phosphate having a high water retention capacity for the catalyst layer (see, for example, Patent Documents 1 to 7). Thereby, the output can be sufficiently maintained over a long period of time even under low humidification conditions. However, it is thought that proton conductivity alone was insufficient with zirconium phosphate alone, but polymer electrolyte material was also used in the catalyst layer at the same time, so this technology uses operating temperature, resin degradation, catalyst paste The stability problem has not been solved.

  In addition, it has been proposed to use zirconium phosphate for the electrolyte membrane and the catalyst layer (see, for example, Patent Document 2). As a result, the attack of OH radicals on the polymer electrolyte material is suppressed, and the durability thereof is enhanced. Therefore, the fuel cell performance can be maintained and operation can be performed for a long time. However, even in this technique, since the polymer electrolyte material is used for the catalyst layer, the problems of the operating temperature and the stability of the catalyst paste cannot be solved.

  It has also been proposed to use an electrolyte membrane in which a polymer electrolyte material and zirconium phosphate are combined (see, for example, Patent Document 3). According to this technology, it is possible to operate at 90 (° C.) higher than before, and good power generation characteristics can be obtained even under low humidity. However, since a polymer electrolyte material is used for the catalyst layer, the problems of resin degradation and catalyst paste stability cannot be solved.

  In addition, it has been proposed to use tin oxide as an electron conductor and catalyst carrier of the catalyst layer, provide tin phosphate having proton conductivity on the surface, and use proton conductive indium phosphate as an inorganic binder (for example, (See Patent Document 4). According to this technology, it is not shown how much power generation characteristics can be obtained, but since the catalyst layer is composed of an inorganic material, the deterioration of the catalyst layer described above is suppressed and stable over the long term. Cell characteristics are obtained. In addition, since the catalyst layer does not contain a resin, there is a possibility that high characteristics can be obtained even in a low humidification or high temperature environment depending on the selection of the electrolyte membrane constituent material. However, the preparation process of the catalyst paste is complicated, and includes a heat treatment step, and an expensive indium compound is used as the inorganic binder, so that there is a problem in terms of manufacturing cost.

  As described above, conventionally, attempts have been made to enable operation at low humidification and high temperatures by using zirconium phosphate, tin phosphate, etc., and to suppress deterioration of the resin in the catalyst layer and changes in the resin paste. However, none of them was satisfactory, and the cost was not satisfactory.

  The present invention has been made in the background of the above circumstances, and its purpose is to form a catalyst layer that can be operated at low humidification and high temperature and hardly deteriorates at low cost, and has excellent stability. It is to provide a method for preparing a paste.

  In order to achieve such an object, the gist of the present invention is to manufacture a porous catalyst layer provided in a state in which a gas can be guided on a solid polymer electrolyte in order to constitute a solid polymer fuel cell. A method for preparing a catalyst paste for the purpose of suspension dropping, in which a dispersion of a fine carrier carrying a catalyst dispersed in a solvent is stirred and a zirconium phosphate gel suspension is dropped into the dispersion It includes the steps.

  According to this configuration, the zirconium phosphate gel suspension is dropped into the catalyst-supporting carrier dispersion while stirring, so that the zirconium phosphate gel is uniformly dispersed in the dispersion. Therefore, when the catalyst paste thus obtained is applied to a gas diffusion electrode or an electrolyte membrane and dried to form a catalyst layer, a catalyst layer in which zirconium phosphate, which is an inorganic electrolyte material, is uniformly dispersed is obtained. Therefore, sufficiently high proton conductivity can be obtained with only this zirconium phosphate without hindering gas diffusion in the catalyst layer, so that it is not necessary to use a polymer electrolyte material such as perfluorocarbon sulfonic acid for the catalyst layer. Therefore, it is possible to form a catalyst layer that can be operated even under low humidification or high temperature and hardly deteriorates, and a catalyst paste having excellent stability can be obtained. The zirconium phosphate gel suspension is a liquid obtained by stirring and dispersing zirconium phosphate gel in water.

Here, preferably, in the suspension dropping step, zirconium phosphate (Zr (HPO ₄ ) ₂ · nH ₂ O) is 10 (wt%) or more and 50 (wt%) with respect to the total solid content in the paste. The zirconium phosphate gel suspension is dropped so as to have a ratio within the following range. In this way, since the amount of zirconium phosphate added is in an appropriate range, when the catalyst layer is formed using this catalyst paste when manufacturing a fuel cell, particularly high power generation characteristics can be obtained. For example, a large current density of 300 (mA / cm ² ) or more can be obtained at 0.4 (V). If the amount of zirconium phosphate is 10 (wt%) or more, the proton conduction path is sufficiently formed, and if it is 50 (wt%) or less, the decrease in the electron conduction path of the catalyst layer and the decrease in electron conductivity The decrease in gas diffusivity is kept small enough.

Preferably, the catalyst paste preparation method includes a suspension synthesis step of synthesizing the zirconium phosphate gel suspension by mixing phosphoric acid with the zirconium precursor powder and reacting the mixture. In this way, a zirconium phosphate gel suspension that is easily dispersed when dropped into the dispersion in the suspension dropping step can be obtained, thereby forming a catalyst layer that can further enhance the power generation characteristics of the fuel cell. A possible catalyst paste is obtained. For example, zirconium hydroxide Zr (OH) ₄ can be suitably used as the zirconium precursor powder.

Preferably, the catalyst paste preparation method includes a precursor synthesis step of synthesizing the zirconium precursor powder by mixing and reacting a chelating agent with a zirconium solution. In this way, when preparing a zirconium phosphate gel suspension, a zirconium precursor powder that is easily dispersed in water is obtained. For example, zirconium butoxide can be preferably used as the zirconium solution, and acetylacetone can be preferably used as the chelating agent. Thereby, the zirconium precursor Zr (OH) ₄ described above can be easily obtained.

  Preferably, the fine carrier is carbon particles. The constituent material and shape of the carrier are not particularly limited. In addition to carbon, conductive metal oxides, corrosion-resistant metals, etc. can be used. In addition to particles, fibrous materials can also be used. It is preferable to use carbon particles in view of handling and ease of handling. Further, it is preferable to use particles having an average particle diameter of about 30 (nm).

  In addition, when using the catalyst paste obtained by the preparation method of the present invention, for example, a catalyst layer can be formed by applying on a gas diffusion electrode and performing a drying treatment. For example, what is necessary is just to determine so that a desired catalyst carrying amount may be obtained. Further, after forming the catalyst layer in this manner, two electrodes with the catalyst layer are stacked with the polymer electrolyte membrane interposed therebetween, and heated while being pressed to obtain a flat plate MEA. The MEA to which the catalyst paste preparation method of the present invention is applicable is not limited to a flat plate shape, and may be a cylindrical shape.

The catalyst paste obtained by the catalyst paste preparation method of the present invention is applied to a solid polymer fuel cell using various solid polymer electrolytes, and the material of the solid polymer electrolyte is not particularly limited. Examples of the solid polymer electrolyte include a homopolymer or copolymer of a monomer having an ion exchange group (such as a -SO ₃ H group), a monomer having an ion exchange group, and another monomer copolymerizable with the monomer. Copolymer, a homopolymer of a monomer having a functional group that can be converted to an ion exchange group by post-treatment such as hydrolysis (that is, a precursor functional group of an ion exchange group), or a copolymer (proton conductive polymer) For example, a precursor obtained by performing a similar post-treatment.

  Specific examples of the polymer electrolyte include, for example, perfluoro proton conductive polymer such as perfluorocarbon sulfonic acid resin, perfluorocarbon carboxylic acid resin film, sulfonic acid type polystyrene-graft-ethylenetetrafluoroethylene (ETFE). Copolymer membrane, sulfonic acid type poly (trifluorostyrene) -graft-ETFE copolymer membrane, polyetheretherketone (PEEK) sulfonic acid membrane, 2-acrylamido-2-methylpropanesulfonic acid (ATBS) membrane, carbonized Examples include hydrogen-based films.

It is a schematic diagram which shows the structure of the flat plate type MEA using the catalyst paste prepared by the method of this invention. It is process drawing explaining the zirconium precursor synthetic | combination process among the catalyst paste preparation methods for forming the catalyst layer of FIG. It is process drawing explaining the process of preparing a zirconium phosphate gel suspension using the zirconium precursor obtained at the process of FIG. It is process drawing explaining the process of preparing a catalyst paste using the zirconium phosphate gel suspension obtained at the process of FIG. It is a figure which shows the result of having changed the addition amount in the suspension addition process of FIG. 4, and evaluating the relationship between zirconium phosphate content in a catalyst paste, and a power generation characteristic. It is a figure which shows typically the structure of a fuel cell evaluation apparatus.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In the following embodiments, the drawings are appropriately simplified or modified, and the dimensional ratios, shapes, and the like of the respective parts are not necessarily drawn accurately.

  FIG. 1 is a diagram showing a cross-sectional structure of a flat plate MEA 10 manufactured using a catalyst paste according to the preparation method of one embodiment of the present invention. In FIG. 1, the MEA 10 includes a thin flat layer electrolyte membrane 12, catalyst layers 14 and 16 provided on both surfaces thereof, and gas diffusion electrodes 18 and 20 provided on the upper surfaces of the catalyst layers 14 and 16. Has been.

  The electrolyte membrane 12 is made of a proton conductive polymer such as Nafion (registered trademark of DuPont), and has a thickness dimension in the range of 20 to 200 (μm), for example, about 50 (μm). I have.

  The catalyst layers 14 and 16 include, for example, Pt-supported carbon black (hereinafter referred to as Pt / C) in which a catalyst such as platinum is supported on spherical carbon powder, and zirconium phosphate which is a proton conductive inorganic material. A porous layer. In the catalyst layers 14 and 16, zirconium phosphate is contained in a range of 10 to 50 (wt%), for example, about 20 (wt%). The catalyst layers 14 and 16 do not contain any organic compound such as Nafion. Further, as Pt / C, for example, those commercially available from Tanaka Kikinzoku Kogyo Co., Ltd. are used. The thickness of the catalyst layers 14 and 16 is, for example, about 50 (μm).

  The gas diffusion electrodes 18 and 20 are made of carbon fiber paper (carbon paper) having a thickness of about 380 (μm), for example, and a porous layer through which gas can easily flow in the thickness direction. It is. As this carbon fiber paper, for example, those commercially available for fuel cells from Toray Industries, Inc. are used.

The MEA 10 configured as described above has sufficiently high battery characteristics such as a current density of 430 to 720 (mA / cm ² ) when the voltage between the gas diffusion electrodes 18 and 20 is 0.4 (V). is doing. Further, as described above, since the catalyst layers 14 and 16 do not contain an organic compound, the deterioration of the organic compound, the limitation of the low-humidity environment and the operation in the middle temperature range, etc. are alleviated. That is, inconveniences when the polymer electrolyte is used for the catalyst layers 14 and 16 are avoided while ensuring sufficient battery characteristics.

  2 to 4 are process diagrams illustrating a method for preparing a catalyst paste for forming the catalyst layers 14 and 16 when the MEA 10 is manufactured. FIG. 2 shows a process for synthesizing a zirconium precursor, FIG. 3 shows a process for preparing a zirconium phosphate gel suspension, and FIG. 4 shows a process for preparing a catalyst paste. In the present embodiment, the preparation process of the catalyst paste for forming the catalyst layers 14 and 16 is divided into such three stages.

  In FIG. 2 described above, in step 1, a zirconium solution is mixed with a solvent. As this solvent, for example, 2-propanol is used. As the zirconium solution, for example, a solution in which zirconium butoxide is dissolved in 1-butanol at a concentration of 85 (%) is used. Next, in step 2, a chelating agent is further mixed with this. As the chelating agent added here, for example, acetylacetone is used. Next, in Step 4, 1N nitric acid is added to this as a catalyst. The preparation amounts in these steps 1 to 4 were, for example, 2-propanol 400 (g), zirconium solution 22.6 (g), acetylacetone 10 (g), and nitric acid 20 (ml).

After all mixing, in the mixing step of step 4, this is mixed, for example, with a magnetic stirrer, for example, at 400 rpm for 3 hours. Thereby, the zirconium precursor Zr (OH) ₄ is produced by the reaction of zirconium butoxide and acetylacetone. Next, in the drying step of Step 5, the mixed solution is dried at 80 (° C.), for example. The drying time is, for example, about 18 hours. Thereby, a zirconium precursor powder is obtained.

  Next, in step 6 shown in FIG. 3, the zirconium precursor powder prepared as described above is prepared, and in step 7, purified water is mixed therewith. Next, 1N nitric acid is added to this for pH adjustment. These preparation amounts were, for example, zirconium precursor powder 1.2 (g), purified water 13.2 (g), and nitric acid 0.84 (g). In the mixing step of Step 8, these are mixed for 60 minutes, for example, at 400 rpm with a magnetic stirrer, for example, and the zirconium precursor powder is dispersed in water. Next, in step 10, 85 (%) phosphoric acid is added thereto, and in the mixing and reaction step of step 11, this is further stirred and mixed to react the zirconium precursor and phosphoric acid to form zirconium phosphate. Leave at room temperature overnight. The amount of phosphoric acid added was, for example, 3.46 (g).

  Next, in the water washing step of step 12, after the reacted liquid mixture is filtered, excess phosphoric acid is washed away with water. Next, in the mixing step of step 13, the produced zirconium phosphate gel is mixed with purified water. Thereby, a zirconium phosphate gel suspension is obtained.

  Next, in step 14 shown in FIG. 4, a Pt / C catalyst is prepared. Next, in step 15, this is dispersed in purified water, and in step 16, a solvent is added thereto. The Pt / C catalyst is commercially available and contains, for example, 45.8 (%) of Pt. The solvent is 2-propanol, for example. These preparation amounts were, for example, Pt / C catalyst 0.4 (g), purified water 0.8 (g), and solvent 2.8 (g). Thereby, a dispersion in which the Pt / C catalyst is dispersed in the solvent is obtained.

  Next, in step 17, the above dispersion is mixed and stirred. This processing is performed, for example, by rotating the rotor at a rotational speed of about 400 (rpm) within a range of 200 to 500 (rpm) using a magnetic stirrer. After mixing and stirring for 10 minutes, for example, particles are dispersed by applying ultrasonic vibration to the container containing the paste in step 18 with an ultrasonic cleaner for 10 minutes, for example. Subsequently, in step 19, as in step 17, stirring using a magnetic stirrer is performed. Then, with the stirring continued, in step 20, the zirconium phosphate gel suspension is dropped into the dispersion using a pipette or the like. The dripping amount of the zirconium phosphate gel suspension is, for example, 10 to 50 (wt%) of the total amount of the solid content, that is, the Pt / C catalyst. In step 21, mixing and stirring are continued for 10 minutes after completion of the dropping. Also in this step 21, it is preferable to disperse the particles by applying ultrasonic vibration for about 10 minutes after stirring for 10 minutes using, for example, a magnetic stirrer. Thereby, a catalyst paste in which the zirconium phosphate gel suspension is uniformly dispersed is obtained.

In this way, a catalyst paste is prepared, applied to carbon paper, dried at room temperature overnight, for example, and then the catalyst layer is placed inside between two carbon papers each provided with a catalyst layer. Sandwich the electrolyte membrane made of Nafion etc. and hot press. The application amount of the catalyst paste is, for example, an amount corresponding to a Pt amount of 0.25 (mg / cm ² ), and is performed by an appropriate method such as brush coating or dip coating. The hot press is performed, for example, at a temperature of about 130 (° C.), a pressure of about 5.3 (MPa), and a heating and pressing time of 1 minute. Thereby, the MEA 10 is obtained.

  Therefore, according to this example, since the zirconium phosphate gel suspension was dropped into the Pt / C catalyst dispersion while stirring, a catalyst paste in which the zirconium phosphate gel suspension was uniformly dispersed was obtained. Since the catalyst layers 14 and 16 are formed using such a catalyst paste, a structure in which zirconium phosphate is uniformly dispersed is obtained. As a result, proton conductivity is enhanced while sufficiently maintaining electron conductivity and gas diffusivity, and it is considered that the high battery characteristics as described above can be obtained without using a polymer electrolyte.

  Moreover, according to the present Example, since an organic compound is not contained in a catalyst paste, there also exists an advantage which is excellent in stability.

  Table 1 and FIG. 5 below show the battery characteristics of the MEA 10 by changing the dripping amount of the zirconium phosphate gel suspension when preparing the catalyst paste in the MEA 10 manufacturing process in a range of up to 60 (wt%). The results of measurement are summarized. The production conditions of each sample were the same as described above except that the amount of the zirconium phosphate gel suspension was different, and thus the zirconium phosphate contents in the catalyst layers 14 and 16 were different. In Table 1, “ZrP introduction amount” represents the amount of solid zirconium phosphate in the dropped zirconium phosphate gel suspension as a percentage of the total solid content in the catalyst paste. “OCV” is the open circuit voltage, and “current density” is the current density when the voltage is 0.4 (V) and 0.6 (V).

Further, the current density was measured using a fuel cell evaluation apparatus 30 whose schematic configuration is shown in FIG. In FIG. 6, for example, hydrogen is supplied from the fuel supply source 32 to the anode-side gas diffusion electrode 18 through the anode-side humidifying tank 34 as fuel. On the electrode 18 to which hydrogen and water are supplied, an oxidation reaction of the following formula (1) occurs, and protons H ⁺ and electrons e ⁻ are generated. Protons flow through the electrolyte layer 12 toward the catalyst layer 16 on the cathode side, and electrons are extracted from a terminal (not shown) connected to the electrode 18 and flow to the load 36 via an external circuit. The electrons supplied to the load 36 further travel to the catalyst layer 16 via an external circuit. Then, on the catalyst layer 16, protons and electrons undergo a reduction reaction of the following formula (2) with oxygen supplied from the oxygen supply source (or air supply source) 38 via the cathode side humidification tank 40. generate. Note that methanol may be supplied on the fuel side instead of hydrogen, and the oxidation reaction in that case is shown in the following equation (3).
_{^{3H 2 → 6H + + 6e -}} ··· (1)
^{_{3 / 2O 2 + 6H + +}} 6e - → 3H 2 O ··· (2)
_{CH 3 OH + H 2 O →} 6H + + CO 2 + 6e - ··· (3)

  The current densities shown in Table 1 and FIG. 5 are as follows. In the evaluation apparatus 30, the temperature of the cell (MEA) 10, the humidifying tanks 34 and 40, the anode gas is 70 (° C.), and the hydrogen flow rate (anode side) is 0.2. (l / min), keeping the air flow rate (cathode side) at 1.5 (l / min), adjusting the load 36, measuring the terminal voltage while changing the current density, and calculating from the current density and terminal voltage It is the maximum value.

As shown in Table 1 and FIG. 5, when the amount of zirconium phosphate is in the range of 10 to 50 (wt%), 0.4 (V) is 430 to 720 (mA / cm ² ), 0.6 (V) is 140 to A current density of 210 (mA / cm ² ) was obtained. Comparing the curves of 0 (wt%) and 60 (wt%) with the other curves of 10-50 (wt%), the current density is dramatically improved with a slight addition of zirconium phosphate. wt%) shows a maximum value, and when added excessively, the current density tends to decrease. Up to 50 wt%, the current density improvement effect is clear.

  When zirconium phosphate is added, it forms a proton path in the catalyst layers 14 and 16, but in the range of 10 to 50 (wt%), there is a good balance between electron conductivity and gas diffusivity and proton conductivity. Is considered to be obtained. Although the power generation characteristics are remarkably improved by the addition of 10 (wt%), this suggests that a proton path is formed in the catalyst layers 14 and 16 by zirconium phosphate. The reason why the power generation characteristics show a tendency to decrease when the amount added is large is that when the amount of zirconium phosphate increases, the contact between carbon blacks is hindered and the electron conductivity decreases, and the zirconium phosphate inhibits gas diffusion. It is guessed.

  As mentioned above, although this invention was demonstrated in detail with reference to drawings, this invention can be implemented also in another aspect, A various change can be added in the range which does not deviate from the main point.

10: MEA, 12: electrolyte membrane, 14, 16: catalyst layer, 18, 20: gas diffusion electrode

Claims (4)

A method for preparing a catalyst paste for producing a porous catalyst layer provided in a state in which a gas can be guided on a solid polymer electrolyte to constitute a solid polymer fuel cell,
Preparation of catalyst paste characterized by including a suspension dropping step of dropping a zirconium phosphate gel suspension into a dispersion while stirring a dispersion in which a fine carrier carrying a catalyst is dispersed in a solvent Method.   In the suspension dropping step, the zirconium phosphate gel suspension is dropped so that the zirconium phosphate has a ratio in the range of 10 (wt%) to 50 (wt%) with respect to the total solid content in the paste. The method of preparing a catalyst paste according to claim 1.   3. The method of preparing a catalyst paste according to claim 1, further comprising a suspension synthesis step of synthesizing the zirconium phosphate gel suspension by mixing phosphoric acid with the zirconium precursor powder and reacting the mixture.   The catalyst paste preparation method according to claim 3, comprising a precursor synthesis step of synthesizing the zirconium precursor powder by mixing and reacting a chelating agent with a zirconium solution.

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