Title of Invention: ZINC BASED CATALYST PARTICLE HAVING CORE-SHELL STRUCTURE AND METHANATION METHOD OF CARBON DIOXIDE USING THE SAME

Technical Field

The present invention relates to a zinc based catalyst particle having a core-shell structure and a methanation method of carbon dioxide using the same, and more particularly, to a zinc based catalyst particle having a core-shell structure, which has excellent photoactivity and methanation activity for carbon dioxide, and a methanation method of carbon dioxide using the same.

Background Art

[2] Recently, environmentally, as global warming is gradually becoming a serious issue, an increase in concentration of carbon dioxide, which is one of the main causes of the greenhouse effect, has been significantly highlighted. In relation to the problem as described above, various efforts to capture and store carbon dioxide, and the like, have been attempted, but the ultimate solution is to allow carbon dioxide to serve as a useful material by directly using carbon dioxide. Therefore, in view of entirely managing carbon dioxide, utilization and reduction of carbon dioxide will be the ultimate goal.

However, there are various difficulties in overcoming carbon-oxygen bond energy in a molecule in order to chemically utilize carbon dioxide, and a high temperature, large electrochemical energy, and the like, have been required.

Currently, various conversion methods for converting carbon dioxide to organic materials capable of being variously utilized through a thermochemical method, an electrochemical method, a biochemical method, a photochemical method, and the like, have been studied. Among them, a photochemical reaction using a photocatalyst has advantages in that a reaction condition to be required is relatively low as compared to other methods, and solar energy corresponding to an energy source, which is the most prominent renewable energy unlike fossil fuel directly linked with environmental problems, may be eco-friendly and be continuously used. Further, a photoreduction method of carbon dioxide using the photocatalyst has excellent advantages in that effective conversion to useful and economical compounds such as methane, methanol, ethanol, and the like, may be performed by precisely adjusting a reaction route without a selective separation process of carbon dioxide.

However, many scientists have made efforts on the photoreduction method of carbon dioxide using this photocatalyst, but conversion efficiency was still low. Therefore, it is expected that development of a catalyst material having high conversion efficiency for a photochemical reduction reaction of carbon dioxide and establishment of suitable reaction conditions may significantly contribute to solving global environmental problems and developing science and technology, and the like.

[6] A catalyst for the photochemical reduction reaction of carbon dioxide, according to related art is as follows.

[7] A method of preparing methane and methanol using nano-sized titanium dioxide particles with high efficiency has been disclosed in Non-Patent Document 1(J. Phys. Chem. B Vol 91 , 4305 ( 1987)), but there is a problem in that since the photoreduction reaction of carbon dioxide is accompanied with decomposition of water, which is used as a reduction agent, a yield is significantly low. Therefore, a method of using cavities of zeolite in order to design and apply a high-efficiency and high-selectivity photo- catalyst system has been suggested, and as a method of adjusting a size of a titanium dioxide crystal particle, after dispersing an aqueous titanium ammonium oxalate solution in Y-zeolite having a constant sized pore and an inlet using an ion exchange method and an impregnation method, and applying the resultant to a photoreduction reaction of carbon dioxide, an influence of dispersion of the titanium dioxide crystal particle on reaction activity was observed, and the result was reported in Non-Patent Document 2(Stud. Surf. Sci. Catal., Vol. 114, 177, ( 1998)). Here, it was shown that the titanium dioxide crystal particle prepared using the ion exchange method was more suitably dispersed as compared to the titanium dioxide crystal particle prepared using the impregnation method, such that the photoreduction reaction activity for carbon dioxide was improved, and it was observed that in a case of increasing a concentration of the aqueous titanium ammonium oxalate solution using the impregnation method, the titanium dioxide crystal particles were aggregated, and thus the photoreduction reaction activity was rather deteriorated. However, in this case, conversion efficiency was also low, such that this method is not suitable for commercialization.

[8] As another method, a method of preparing titanium dioxide sol in nitric acid and titanium tetraisopropoxide in ethanol and supporting the prepared titanium dioxide sol on a supporter such as ZSM-5, zeolite- A, alumina, silica, or the like, and preparing titanium dioxide powder in which anatase-, rutile-, and brookite-forms were mixed with each other by sintering has been disclosed in Patent Document 1(U.S. Patent No. 5,981 ,426). A method of preparing a titanium dioxide photocatalyst, capable of removing hazardous gas ingredients such as ammonia, nitrogen oxides, sulfides, aldehydes, volatile organic compounds, chlorine based volatile organic compounds, or the like, in the air in a UV region, organic materials in water, and the like, with high quantum efficiency, which is 20 to 100 times higher than that in the related art, by injecting citric acid and isopropanol into titanium tetraisopropoxide in isopropanol, adding ethylene glycol thereto under a mild acidic solution condition to prepare titanium dioxide sol in which titanium dioxide powder having a size smaller than 5 nm is uniformly dispersed, adsorbing the prepared titanium dioxide sol in cavities of various kinds of zeolite carrier skeletons, and then uniformly dispersing the titanium dioxide powder by sintering at a size of 5 to 20 nm, which is smaller than the existing size (30nm)(Patent Document 2. Korean Patent Laid-Open Publication No.

10-2002-0041604). Further, a method of reacting carbon monoxide, carbon dioxide, or mixed gas thereof with hydrogen gas in the presence of an iron group transition element to prepare methane gas has been disclosed in Patent Document 3 (Japanese Patent No. 5691119), but this method has a disadvantage in that a yield of the methane gas is low.

[9] As described above, in order to solve problems of the reaction using the photo- catalyst for the photochemical reduction reaction of carbon dioxide according to the related art, the present applicant tried to provide a zinc based catalyst particle having a core-shell structure, which has excellent photoactivity and methanation activity for carbon dioxide, and a methanation method of carbon dioxide using the same, capable of producing methane at a high production rate, thereby completing the present invention.

Disclosure of Invention

Technical Problem

[10] An object of the present invention is to provide a zinc based catalyst particle having a core-shell structure and a high-efficiency methanation method of carbon dioxide using the same.

Solution to Problem

[1 1] According to an exemplary embodiment of the present invention, there is provided a catalyst particle having a core-shell structure, including a zinc oxide core and a metal shell particle enclosing the core, wherein a core diameter Dp and a characteristic length Lc of the metal shell particle satisfy the following Equation 1, a ratio of a surface area of an exposed zinc oxide core that is not treated with the metal shell particle to an entire surface area of the catalyst particle is in a range of 0.54 to 0.94:

[12] [Equation 1]

[13] 0.30 < Lc/Dp < 0.45.

[14] The metal shell particle may be represented by the following Structure:

[15] MxOy

[16] [where,

[17] M is magnesium, calcium, strontium, barium, titanium, chromium, gallium,

germanium, yttrium, zirconium, molybdenum, silver, cadmium, indium, tin, platinum, gold, lead, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, ytterbium, lutetium, copper, nickel, cobalt, iron, ruthenium, manganese, tungsten, or vanadium;

[18] x is an integer of 1 to 3; and

[19] y is an integer of 1 to 5;

[20] x and y satisfying 0.1 < x/y < 3.0].

[21] The metal shell particle may be NiO, Cu ₂ 0, CuO, CoO, Co ₂ 0 ₃ , Co ₃ 0 ₄ , FeO, Fe ₂ 0 ₃ ,

Fe ₃ 0 ₄ , Ru ₂ 0, W0 ₃ , or V ₂ O _s .

[22] An average particle size of the catalyst particle may be 5 to 500 nm.

[23] The catalyst particle may have photocatalytic activity.

[24] According to another exemplary embodiment of the present invention, there is

provided a method of converting carbon dioxide to methane using a catalyst particle having a core-shell structure including a zinc oxide core.

[25] The catalyst particle may include a metal shell particle having the following

structure:

[26] M _x O _y

[27] [Where,

[28] M is magnesium, calcium, strontium, barium, titanium, chromium, gallium,

germanium, yttrium, zirconium, molybdenum, silver, cadmium, indium, tin, platinum, gold, lead, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, ytterbium, lutetium, copper, nickel, cobalt, iron, ruthenium, manganese, tungsten, or vanadium;

[29] x is an integer of 1 to 3; and

[30] y is an integer of 1 to 5;

[31] x and y satisfying 0.1 < x/y < 3.0].

[32] The metal shell particle may be NiO, Cu ₂ 0, CuO, CoO, Co ₂ 0 ₃ , Co ₃ 0 ₄ , FeO, Fe ₂ 0 ₃ , Fe ₃ 0 ₄ , Ru ₂ 0, W0 ₃ , or V ₂ 0 ₅ .

[33] The catalyst particle may be characterized in that a core diameter D _p of the catalyst particle and a characteristic length L _t of the metal shell particle satisfy the following Equation 1 , a ratio of a surface area of an exposed zinc oxide core that is not treated with the metal shell particle to an entire surface area of the catalyst particle is in a range of 0.54 to 0.94:

[34] [Equation 1]

[35] 0.30 < L/D _p < 0.45.

[36] The conversion may be a photochemical conversion.

[37] A light source of the photochemical conversion may have a wavelength of 250 to 500 nm.

Advantageous Effects of Invention [38] The catalyst particle according to the present invention has excellent photoactivity and methanation activity for carbon dioxide, and the catalytic activity may be optimally increased by adjusting the characteristics of the core-shell structure.

[39] In addition, the catalyst particle according to the present invention has excellent methanation activity for carbon dioxide, such that the catalyst particle has advantages in that it is possible to decrease carbon dioxide in the air due to excessive uses of fossil fuel and re-circulate carbon dioxide as high energy fuel.

Brief Description of Drawings

FIG. 1 briefly illustrates a synthesis method of a catalyst particle having a core-shell structure prepared in Example 1 according to the present invention.

FIG. 2 illustrates a transmission electron microscope (TEM) photograph of the catalyst particle prepared in Example 1 according to the present invention.

FIG. 3 illustrates an X-ray diffraction (XRD) pattern of the catalyst particle prepared in Example 1 according to the present invention.

FIG. 4 is a graph illustrating reactivity of the catalyst particle at the time of conversion of carbon dioxide using the catalyst particle prepared in Example 1 according to the present invention.

[44] FIG. 5 is a graph illustrating methane production amounts of catalyst particles at the time of conversion of carbon dioxide using the catalyst particles prepared in Example 1 according to the present invention and Comparative Examples 1 to 4.

Best Mode for Carrying out the Invention

A zinc based catalyst particle having a core-shell structure according to the present invention and a methanation method of carbon dioxide using the same will be described below. Here, technical terms and scientific terms used in the present specification have the general meaning understood by those skilled in the art to which the present invention pertains unless otherwise defined, and a description for the known function and configuration unnecessarily obscuring the gist of the present invention will be omitted in the following description.

The present applicant conducted studies on a zinc based catalyst particle, and as a result, the present applicant confirmed that catalytic activity may be rapidly changed depending on a shape, a constitution material, and a content of the zinc based catalyst particle. In addition, the present applicant conducted further studies, and as a result, the present applicant confirmed that when the zinc based catalyst particle has a core-shell structure, the zinc based catalyst particle has higher photoactivity.

Further, the present applicant found that the zinc based catalyst particle having particularly the core-shell structure may be used as a photocatalyst capable of reducing carbon dioxide to convert carbon dioxide to methane with high selectivity, and in a case in which a specific parameter is satisfied, the photoactivity was rapidly increased, thereby completing the present invention.

[48] The present invention provides a zinc based catalyst particle having a core-shell structure, which is a photocatalyst material having excellent chemical and biological stability and durability, and having high selectivity at the time of photochemical reduction reaction of carbon dioxide, as a material having excellent photoactivity by light in a UV region.

[49] The catalyst particle according to an exemplary embodiment of the present invention has a core-shell structure including a zinc oxide core and a metal shell particle enclosing the core, and catalytic activity may be adjusted by adjusting characteristics of the core-shell structure as described above.

[50] In detail, in the catalyst particle having a core-shell structure, photocatalytic activity may be adjusted by adjusting a core diameter D _p of the zinc oxide core, which is a photocatalyst metal oxide, a characteristic length L _c of the metal shell particle, and the like, and a plasmonic phenomenon is maximized by interaction between the zinc oxide core and the metal shell particle occurring when the zinc oxide core and the metal shell particle receive light energy, such that the photocatalytic activity may be maximized.

[51] In the present specification, the term "core diameter D _p " indicates an average

diameter of zinc oxide corresponding to the core of the catalyst particle, and the term "characteristic length L _c " indicates an average thickness (height) of the shell particle treated with the metal shell particle.

[52] In the catalyst particle according to the exemplary embodiment of the present

invention, the core diameter D _p and the characteristic length L _c of the metal shell particle satisfy Equation 1 , such that the photocatalytic activity may be rapidly improved:

[Equation 1]

0.30 < L/D _p < 0.45.

In detail, the catalyst particle has the core-shell structure including the zinc oxide core and the metal shell particle enclosing the core, and a portion treated with the metal shell particle has the characteristic length measured from a surface of the core, and in a portion that is not treated with the metal shell particle, zinc oxide, which is core particle, may be exposed. The catalytic activity may be optimally adjusted by adjusting a ratio of a surface area of the portion treated with the metal shell particle described above to a surface area of the portion that is not treated with the metal shell particle.

Further, in the catalyst particle according to the exemplary embodiment of the present invention, the ratio of the surface area of the portion that is not treated with the metal shell particle to a surface area of the catalyst particle may be in a range of 0.54 to 0.94, and in order to have a more improved photocatalytic activity, the ratio may be preferably in a range of 0.62 to 0.93, and more preferably, in a range of 0.70 to 0.85.

[57] In a case in which the core diameter D _p and the characteristic length L _c satisfy

Equation 1 as described above and the ratio of the surface area of the portion that is not treated with the metal shell particle is in a range of 0.54 to 0.94, the catalytic particle according to the present invention has an unexpected but excellent photoactivity, and in a case in which specific parameters as described above are out of the range, it is impossible to implement effects to be desired.

[58] Further, it may be appreciated that in the case in which the catalyst particle according to the present invention satisfies the above-mentioned parameters, the photocatalytic activity and selectivity for methane are significantly improved, and in view of rapidly increasing a methane production amount, a range of L/D _p is preferably more than 0.33 to less than 0.40, and more preferably, more than 0.35 to less than 0.39. It is predicted that the effects as described above are characteristic effects exhibited due to interaction between the zinc oxide core and the metal shell particle generated by light, that is, maximization of the plasmonic phenomenon.

[59] In the catalyst particle according to the exemplary embodiment of the present

invention, the metal shell particle may be represented by the following structure:

[60] M _x O _y

[61] [Where,

[62] M is magnesium, calcium, strontium, barium, titanium, chromium, gallium,

germanium, yttrium, zirconium, molybdenum, silver, cadmium, indium, tin, platinum, gold, lead, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, ytterbium, lutetium, copper, nickel, cobalt, iron, ruthenium, manganese, tungsten, or vanadium;

[63] x is an integer of 1 to 3; and

[64] y is an integer of 1 to 5;

[65] x and y satisfying 0.1 < x/y < 3.0].

[66] In the catalyst particle according to the exemplary embodiment of the present

invention, the metal shell particle may be preferably one or more selected among NiO, Cu ₂ 0, CuO, CoO, Co ₂ 0 ₃ , Co ₃ 0 ₄ , FeO, Fe ₂ 0 ₃ , Fe ₃ 0 ₄ , Ru ₂ 0, W0 ₃ , V ₂ 0 ₅ , and the like, in view of ease of handling and low material cost. In view of implementing optimal catalytic activity, it is preferable that the metal shell particle may be one or more selected among NiO, Cu ₂ 0, CuO, CoO, Co ₂ 0 ₃ and Co ₃ 0 ₄ .

[67] An average particle size of the catalyst particle according to the exemplary embodiment of the present invention may be 5 to 500nm, and in view of excellent dispersion force for preventing the catalytic activity from being deteriorated by aggregation of the particle, it is preferable that the catalyst particle has a particle size of 15 to 70 nm. A light source of the catalyst particle according to the exemplary embodiment of the present invention for the photocatalytic activity may be light in a UV region, preferably, light energy having a wavelength of 250 to 500 nm, but is not limited thereto.

Further, the catalyst particle according to the exemplary embodiment of the present invention may be prepared by a method known in the art, for example, a co- precipitation method, a spray method, an electrolysis method, a sol-gel method, a reverse micro-emulsion method, or the like. Hereinafter, a preparation method of the catalyst particle according to the exemplary embodiment of the present invention will be described in detail.

The catalyst particle according to the exemplary embodiment of the present invention may be prepared by a preparation method including: a first step of dissolving a zinc precursor and a binder material in an organic solvent and heating and stirring the zinc precursor solution; and a second step of dissolving a metal precursor in the organic solvent, injecting the metal precursor solution in the zinc precursor solution, and heating and stirring the mixture. In this case, it is preferable that the steps of the preparation method are performed under inert conditions and gas atmosphere such as nitrogen, argon, helium, or the like.

The zinc precursor according to the exemplary embodiment of the present invention is not particularly limited as long as it forms zinc oxide. Non-restrictive examples thereof may include zinc citrate dihydrate, zinc acetate, zinc acetate dihydrate, zinc acetylacetonate hydrate, zinc acrylate, zinc chloride, zinc diethyldithiocarbamate, zinc dimethyldifhiocarbamate, zinc fluoride, zinc fluoride hydrate, zinc hexafluoroacety- lacetonate dihydrate, zinc methacrylate, zinc nitrate hexahydrate, zinc nitrate hydrate, zinc trifluoromethanesulfonate, zinc undecylenate, zinc trifluoroacetate hydrate, zinc tetrafluoroborate hydrate, zinc perchlorate hexahydrate, hydrates thereof, and the like. Preferably, the zinc precursor may be one or more selected from zinc acetate, zinc acetate dihydrate, zinc acetylacetonate hydrate, hydrates thereof, and the like.

In addition, as a specific example of the binder material, one or more selected among polyvinyl pyrrolidone (PVP), polyethylene glycol octylphenyl ether, polyethylene glycol nonylphenyl ether, polyethylene glycol dodecylphenyl ether, polyethylene glycol alkylarylether, polyethylene glycol oleyl ether, polyethylene glycol lauryl ether, polyethylene glycol alkylphenyl ether, polyethylene glycol olefinic acid ether, polyethylene glycol distearic acid ether, polyethylene glycol sorbitan monolaurate, polyethylene glycol sorbitan monostearate, polyethylene glycol alkyl ether, poly- oxyethylene lauryl alcohol ether, polyoxyethylene lauryl fatty acid ester, and the like, may be used. Among them, preferably, the binder material may be selected from polyvinyl pyrrolidone (PVP), polyethylene glycol octylphenyl ether, and the like, and more preferably, polyvinyl pyrrolidone (PVP) having a weight average molecular weight (MW) of 10,000 to 100,000 may be used. Here, the weight average molecular weight (MW) means a value in terms of standard polystyrene, measured using gel permeation chromatography (GPC).

[73] A specific example of the metal precursor may be one or more selected from the

group consisting of dimethyl cadmium, diethyl cadmium, cadmium acetate, cadmium acetylacetonate, cadmium iodide, cadmium bromide, cadmium chloride, cadmium fluoride, cadmium carbonate, cadmium nitrate, cadmium oxide, cadmium perchlorate, cadmium phosphide, cadmium sulfate, lead acetate, lead bromide, lead chloride, lead fluoride, lead oxide, lead perchlorate, lead nitrate, lead sulfate, lead carbonate, tin acetate, tin bisacetylacetonate, tin bromide, tin chloride, tin fluoride, tin oxide, tin sulfate, germanium tetrachloride, trimethyl indium, indium acetate, indium hydroxide, indium chloride, indium oxide, indium nitrate, indium sulfate, dimethyl magnesium, dibutyl magnesium, magnesium ethoxide, magnesium acetylacetonate, magnesium car- boxylate, magnesium halide, nickel acetylacetonate, nickel nitratehexahydrate, nickel acetate tetrahydrate, nickel chloride hexahydrate, cobalt acetate, cobalt acetylacetonate,. cobalt benzoylacetonate, cobalt hydroxide, cobalt bromide, cobalt chloride, cobalt iodide, cobalt fluoride, cobalt cyanide, cobalt nitrate, cobalt sulfate, cobalt selenide, cobalt phosphate, cobalt oxide, cobalt thiocyanate, cobalt propionate, copper nitrate (Cu(N0 ₃ ) ₂ ), copper chloride (CuCl ₂ ), copper sulfate (CuS0 ₄ ), copper acetate ((CH ₃ COO) ₂ Cu), copper(II) acetylacetonate, copper(II) stearate, copper(II) perchlorate, copper(II) ethylenediamine, and copper hydroxide (Cu(OH) ₂ ). More preferably, the metal precursor may be one or more selected from the group consisting of copper nitrate (Cu(N0 ₃ ) ₂ ), copper chloride (CuCl ₂ ), copper sulfate (CuS0 ₄ ), copper acetate ((CH ₃ COO) ₂ Cu), and copper hydroxide (Cu(OH) ₂ ), and most preferably, the metal precursor may be one or more selected among copper nitrate (Cu(N0 ₃ ) ₂ ), copper chloride (CuCl ₂ ), copper acetate ((CH ₃ COO) ₂ Cu), copper hydroxide (Cu(OH) ₂ ), and hydrates thereof.

[74] In this case, the organic solvent used in the preparation method of the catalyst

particle is not limited as long as it may dissolve the zinc precursor, the binder material, and the metal precursor. Preferably, the organic solvent is one or more selected among ethylene glycol, diethylene glycol, triethylene glycol, ethanediol, propanediol, bu- tanediol, pentanediol, formamide, dimethyl formamide, glycerol, dioxane, dimethyl sulfoxide, tetrahydrofuran, and the like, and more preferably, the organic solvent may be one or more selected from ethanediol, propanediol, butanediol, and pentanediol. In addition, for flowability, the organic solvent may be used in a range of 0.001 to 0.01 wt% based on a total weight, but is not limited thereto.

[75] In the preparation method according to the exemplary embodiment of the present invention, 5 to 15 parts by weight of the metal precursor and 20 to 200 parts by weight of the binder material may be used based on 10 parts by weight of the zinc precursor, but are not limited thereto. However, in view of adjusting the characteristics of the core-shell structure to obtain excellent photocatalytic activity, it is preferable that the metal precursor and the binder material are used within the above-mentioned range.

[76] Further, in the preparation method of the catalyst particle according to the exemplary embodiment of the present invention, the heating and stirring may be performed at 150 to 350 °C for 1 to 120 minutes, but is not limited thereto.

[77] In the case of a finally produced catalyst particle, since oxidation rapidly proceeds when the finally produced catalyst particle is exposed to the air, it is preferable that the catalyst particle is put into ethanol having a passivation effect and then stored under a nitrogen atmosphere.

[78] The catalyst particle prepared by the preparation method according to the exemplary embodiment of the present invention may be prepared by further performing a washing step, a drying step, and a sintering step, and impurities such as an organic material, and the like, capable of being contained in the finally produced catalyst particle may be removed by performing these steps, thereby making it possible to obtain more excellent photocatalytic activity.

[79] According to the present invention, the catalyst particle having a core-shell structure including the zinc oxide core may have the photocatalytic activity, and it is possible to convert carbon dioxide to methane with high selectivity by using the catalyst particle.

[80] It was known that in a case of a catalyst used in a photoreduction reaction of carbon dioxide according to the related art, photoreduction reaction activity may be deteriorated due to aggregation of crystal particles, or in the case of using two or more ingredients, the activity may be entirely different depending on a mixing ratio of each of the ingredients.

Therefore, the present applicant tried to prepare the zinc based catalyst particle having a core-shell structure including the zinc oxide core and the metal shell particle enclosing the core to thereby improve photoreduction reaction efficiency of carbon dioxide and selectively improve a production ratio of methane from the photoreduction reaction.

Further, the catalyst particle according to the present invention has excellent long- term stability as compared to a titanium based catalyst according to the related art, and has excellent catalytic activity, such that the catalyst particle may produce methane from carbon dioxide with high selectivity, and the catalytic activity may be adjusted by adjusting the characteristics of the core-shell structure.

The catalyst particle as described above may include the metal shell particle enclosing the zinc oxide core and having the following structure: [84] M _x O _y

[85] [Where,

[86] M is magnesium, calcium, strontium, barium, titanium, chromium, gallium,

germanium, yttrium, zirconium, molybdenum, silver, cadmium, indium, tin, platinum, gold, lead, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, ytterbium, lutetium, copper, nickel, cobalt, iron, ruthenium, manganese, tungsten, or vanadium;

[87] x is an integer of 1 to 3; and

[88] y is an integer of 1 to 5;

[89] x and y satisfying 0.1 < x/y < 3.0].

[90] In a method of converting carbon dioxide to methane according to the present

invention, the catalyst particle may implement a synergic effect between the metal shell particle and the core-shell structure by adjusting the metal shell particle constituting the shell and the core-shell structure.

[91] In a method of converting carbon dioxide to methane according to an exemplary embodiment of the present invention, in view of improving the catalytic activity of the catalyst particle, the metal shell particle may be one or more selected among NiO, Cu ₂ O, CuO, CoO, Co ₂ 0 ₃ , C0 ₃ O4, FeO, Fe ₂ 0 ₃ , Fe ₃ 0 ₄ , Ru ₂ 0, W0 ₃ , V ₂ 0 ₅ , and the like, and more preferably, one or more selected from NiO, Cu ₂ 0, CuO, CoO, Co ₂ 0 ₃ , Co ₃ 0 ₄ , and the like.

[92] Here, in the core-shell structure of the catalyst particle, in view of excellent catalytic activity, it is preferable that the core diameter D _p of the catalyst particle and the characteristic length L _c of the metal shell particle satisfy the following Equation 1. More preferably, the ratio of the surface area of the exposed zinc oxide core that is not treated with the metal shell particle to the entire surface area of the catalyst particle is in a range of 0.54 to 0.94.

[93] [Equation 1]

[94] 0.35 < LJD _p < 0.45

[95] The method of converting carbon dioxide to methane according to an exemplary embodiment of the present invention may be performed by injecting the catalyst particle prepared by the preparation method into an aqueous solution adjusted at pH 6.5 to 7.5. In this case, as a base used to adjust the pH, any base may be used as long as it is used in the art. Specific examples of the base may include sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate, and the like.

[96] Further, in the method of converting carbon dioxide to methane according to the exemplary embodiment of the present invention, the catalyst particle may be mixed in a content range of 0.01 to 0.1 wt% based on a total weight, thereby performing a reduction reaction of carbon dioxide. In this case, the content range of the mixed catalyst particle is not limited thereto, but in a case in which the catalyst particle is mixed within the above-mentioned content range, more excellent photocatalytic activity and methane conversion rate may be implemented, which is preferable.

[97] In the method according to the exemplary embodiment of the present invention, conversion may be a photochemical conversion, and a light source of the photochemical conversion may be light in a UV region, preferably, light energy having a wave length of 250 to 500nm, but is not limited thereto.

[98] The method according to the exemplary embodiment of the present invention may be performed at room temperature (23 °C) and atmospheric pressure, and in order to increase the conversion rate of carbon dioxide to methane, a high-pressure reactor, or the like, which may be used in the art, may also be used.

[99] The present invention will be described in detail with reference to the following

Examples. However, the following Examples are provided only for assisting in the understanding of the present invention, but the scope of the present invention is not limited to the following Examples.

[100] (Example 1 )

[101] a. Synthesis of Zinc Oxide (ZnO)-Copper Oxide (Cu ₂ 0) Catalyst Particle

[102] A zinc precursor solution was prepared by dissolving 0.105 g (0.28 mmol) of a zinc precursor (Zn(acac) ₂ 6H ₂ 0; Zinc(II) acetylacetonate hydrate) and 1 g (9 mmol) of polyvinyl pyrrolidone (PVP) in 40 ml of pentanediol (PD), and the zinc precursor solution was slowly heated to 250 °C for 10 minutes under inert conditions. After the zinc precursor solution was stirred at 250 °C for 3 minutes, a copper precursor solution obtained by dissolving 0.105 g (0.4 mmol) of a copper precursor (Cu(acac) ₂ ;

Copper(II) acetylacetonate) in 5 ml of pentanediol was added thereto. Thereafter, a reaction was carried out at the same temperature for 30 minutes, and a reactant was cooled to room temperature (23 °C). The reactant was washed several times with ethanol and acetone, thereby preparing a zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle (average particle size: 40 nm, yield = 15 %).

[103] FIG. 1 illustrates a shape of the catalyst particle having a core-shell structure

prepared by the preparation method, and in order to confirm a particle structure and a hybrid structure of the catalyst particle, the catalyst particle was analyzed using transmission electron microscope (TEM) and X-ray diffraction (XRD).

[104] As a result, it may be confirmed that the catalyst particle prepared by the method of Example 1 had a core-shell structure as illustrated in FIG. 2, and had a hybrid structure of zinc oxide (ZnO) and copper oxide (Cu ₂ 0) as illustrated in FIG. 3.

[105] Further, distributions of a core diameter D _p of the catalyst particle, a characteristic length L _c of copper oxide, and a ratio of a surface area of an exposed zinc oxide core that was not treated with copper oxide to an entire surface area of the catalyst particle were calculated using a TEM photograph, and the results were illustrated in the following Table 1.

[106] b. Reduction Reaction of Carbon Dioxide Using Catalyst Particle

[107] After 20 ml of 0.2 M sodium carbonate (Na ₂ C0 ₃ ) solution was neutralized to pH 7.4 using a perchloric acid (HC10 ₄ ) solution, 8 mg of the zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle prepared by the method as described above was added thereto. After the solution containing the catalyst particle was transferred to a high-pressure reaction vessel, a reaction system was filled with carbon dioxide gas (pressure = 2.6 bar) by closing an inlet and flowing carbon dioxide gas therein to allow air to be discharged to the outside. In order to sufficiently dissolving carbon dioxide gas, the solution was stirred for 40 minutes. Thereafter, the solution containing the catalyst particle was transferred to a reaction vessel made of a quartz material, and an inlet was closed using a rubber diaphragm. Light was irradiated to the reaction vessel for 3 hours using Xe lamp (300W, Oriel) mounted with a 10cm UV filter as a light source. A gas sample in the reaction vessel was collected using a syringe every one hour during the reaction, and the collected gas sample was analyzed using gas chromatography.

[108] As a result, it may be confirmed that the catalyst particle according to the present invention has a catalyst activation time of I hour, and after the catalyst activation time, reactivity (conversion to methane) was linearly increased (see FIG. 4). Further, it may be appreciated that in a case of using the catalyst particle according to the present invention, selectivity for methane and a methane production amount were excellent (See Table 1).

[109] (Example 2)

[110] After a zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle (average particle size:

40 nm, yield = 15 %) was prepared by the same method as in Example 1 except for using 0.1 10 g (0.42 mmol) of Cu(acac) ₂ (Copper(II) acetylacetonate) as a copper precursor in synthesizing the zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle of Example 1, distributions of a core diameter D _p of the catalyst particle, a characteristic length L _c of copper oxide, and a ratio of a surface area of an exposed zinc oxide core that was not treated with copper oxide to an entire surface area of the catalyst particle were calculated using a TEM photograph, and the results were illustrated in the following Table I.

[111] Further, a reduction reaction was performed by the same method as in Example 1 except for using the zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle prepared by the preparation method of Example 2 instead of the zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle prepared by the preparation method of Example 1 in the reduction reaction of carbon dioxide using the catalyst particle of Example 1, and a conversion rate to methane and a methane production amount were illustrated in Table 1.

[112] (Example 3)

[113] After a zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle (average particle size:

40 nm, yield = 15 %) was prepared by the same method as in Example 1 except for using 0.107 g (0.41 mmol) of Cu(acac) ₂ (Copper(II) acetylacetonate) as a copper precursor in synthesizing the zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle of Example 1, distributions of a core diameter D _p of the catalyst particle, a characteristic length L _c of copper oxide, and a ratio of a surface area of an exposed zinc oxide core that was not treated with copper oxide to an entire surface area of the catalyst particle were calculated using a TEM photograph, and the results were illustrated in the following Table 1.

[114] Further, a reduction reaction was performed by the same method as in Example 1 except for using the zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle prepared by the preparation method of Example 3 instead of the zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle prepared by the preparation method of Example 1 in the reduction reaction of carbon dioxide using the catalyst particle of Example 1, and a conversion rate to methane and a methane production amount were illustrated in Table 1.

[115] (Example 4)

[116] After a zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle (average particle size:

40 nm, yield = 15 %) was prepared by the same method as in Example 1 except for using 0.102 g (0.39 mmol) of Cu(acac) ₂ (Copper(II) acetylacetonate) as a copper precursor in synthesizing the zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle of Example 1 , distributions of a core diameter D _p of the catalyst particle, a characteristic length L _c of copper oxide, and a ratio of a surface area of an exposed zinc oxide core that was not treated with copper oxide to an entire surface area of the catalyst particle were calculated using a TEM photograph, and the results were illustrated in the following Table 1.

[117] Further, a reduction reaction was performed by the same method as in Example 1 except for using the zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle prepared by the preparation method of Example 2 instead of the zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle prepared by the preparation method of Example 1 in the reduction reaction of carbon dioxide using the catalyst particle of Example 1, and a conversion rate to methane and a methane production amount were illustrated in Table 1.

[118] (Example 5)

[119] After a zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle (average particle size:

40 nm, yield = 15 %) was prepared by the same method as in Example 1 except for using 0.097 g (0.37 mmol) of Cu(acac) ₂ (Copper(IT) acetylacetonate) as a copper precursor in synthesizing the zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle of Example 1, distributions of a core diameter D _p of the catalyst particle, a characteristic length L _c of copper oxide, and a ratio of a surface area of an exposed zinc oxide core that was not treated with copper oxide to an entire surface area of the catalyst particle were calculated using a TEM photograph, and the results were illustrated in the following Table 1.

[120] Further, a reduction reaction was performed by the same method as in Example 1 except for using the zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle prepared by the preparation method of Example 2 instead of the zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle prepared by the preparation method of Example 1 in the reduction reaction of carbon dioxide using the catalyst particle of Example 1, and a conversion rate to methane and a methane production amount were illustrated in Table 1.

[121] (Example 6)

[122] After a zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle (average particle size:

40 nm, yield = 15 %) was prepared by the same method as in Example 1 except for using 0.1 15 g (0.44 mmol) of Cu(acac) ₂ (Copper(II) acetylacetonate) as a copper precursor in synthesizing the zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle of Example 1, distributions of a core diameter D _p of the catalyst particle, a characteristic length L _c of copper oxide, and a ratio of a surface area of an exposed zinc oxide core that was not treated with copper oxide to an entire surface area of the catalyst particle were calculated using a TEM photograph, and the results were illustrated in the following Table 1 .

[123] Further, a reduction reaction was performed by the same method as in Example 1 except for using the zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle prepared by the preparation method of Example 2 instead of the zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle prepared by the preparation method of Example 1 in the reduction reaction of carbon dioxide using the catalyst particle of Example 1, and a conversion rate to methane and a methane production amount were illustrated in Table 1.

[124] (Example 7)

[125] a. Synthesis of Zinc Oxide (ZnO)-Nickel Oxide (NiO) Catalyst Particle

[126] A zinc precursor solution was prepared by dissolving 0.105 g (0.28 mmol) of a zinc precursor (Zn(acac) ₂ .6H ₂ 0; Zinc(II) acetylacetonate hydrate) and 1 g (9 mmol) of polyvinyl pyrrolidone (PVP) in 40 ml of pentanediol (PD), and the zinc precursor solution was slowly heated to 250°C for 10 minutes under inert conditions. After the zinc precursor solution was stirred at 250 °C for 3 minutes, a nickel precursor solution obtained by dissolving 0. 105 g (0.41 mmol) of a nickel precursor (Ni(acac) ₂ ; nickel (Π) acetylacetonate) in 5 ml of pentanediol was added thereto. Thereafter, a reaction was carried out at the same temperature for 30 minutes, and the reactant was cooled to room temperature (23 °C). The reactant was washed several times with ethanol and acetone, thereby preparing a zinc oxide (ZnO)-nickel oxide (NiO) catalyst particle (average particle size: 40 nm, yield = 58 %), and a particle structure, a hybrid structure, and a specific surface area of the catalyst particle were measured by the method of Example 1.

[127] As a result, it may be appreciated that the catalyst particle having a hybrid structure of zinc oxide (ZnO) and nickel oxide (NiO), which had a core-shell structure, was formed.

[128] b. Reduction Reaction of Carbon Dioxide Using Catalyst Particle

[129] A reduction reaction was performed by the same method as in Example 1 except for using the zinc oxide (ZnO)-nickel oxide (NiO) catalyst particle prepared by the preparation method as described above instead of the zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle prepared by the preparation method of Example 1 in the reduction reaction of carbon dioxide using the catalyst particle of Example 1 , and a conversion rate to methane was confirmed.

[130] (Example 8)

[131] a. Synthesis of Zinc Oxide (ZnO)-Cobalt Oxide (Co ₃ 0 ₄ ) Catalyst Particle

[132] A zinc precursor solution was prepared by dissolving 0.105 g (0.28 mmol) of a zinc precursor (Zn(acac) ₂ -6H ₂ 0; Zinc(II) acetylacetonate hydrate) and 1 g (9 mmol) of polyvinyl pyrrolidone (PVP) in 40 ml of pentanediol (PD), a d'fhe zinc precursor solution was slowly heated to 250°C for 10 minutes under inert conditions. After the zinc precursor solution was stirred at 250 °C for 3 minutes, a cobalt precursor solution obtained by dissolving 0.1 16 g (0.4 mmol) of a cobalt precursor (Co(N0 ₃ ) ₂ -6H ₂ 0; cobalt (Π) nitrate hexahydrate) in 5 ml of pentanediol was added thereto. Thereafter, a reaction was carried out at the same temperature for 30 minutes, and the reactant was cooled to room temperature (23 °C). The reactant was washed several times with ethanol and acetone, thereby preparing a zinc oxide (ZnO)-cobalt oxide (Co ₃ 0 ₄ ) catalyst particle (average particle size: 40 nm, yield = 93 %), and a particle structure, a hybrid structure, and a specific surface area of the catalyst particle were measured by the method of Example 1.

[133] Further, distributions of a core diameter D _p of the catalyst particle, a characteristic length L _c of cobalt oxide, and a ratio of a surface area of an exposed zinc oxide core that was not treated with cobalt oxide to an entire surface area of the catalyst particle were calculated using a TEM photograph, and the results were conformed.

[134] b. Reduction Reaction of Carbon Dioxide Using Catalyst Particle [135] A reduction reaction was performed by the same method as in Example I except for using the zinc oxide (ZnO)-cobalt oxide (Co ₃ 0 ₄ ) catalyst particle prepared by the preparation method as described above instead of the zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle prepared by the preparation method of Example 1 in the reduction reaction of carbon dioxide using the catalyst particle of Example 1, and a conversion rate to methane was confirmed.

[136] (Comparative Example 1)

[137] A reduction reaction was performed by the same method as in Example 1 except for using zinc oxide as a catalyst particle instead of the zinc oxide (ZnO)-copper oxide (Cu 20) catalyst particle prepared by the preparation method of Example 1 in the reduction reaction of carbon dioxide using the catalyst particle of Example 1 , and a conversion rate to methane and a methane production amount were illustrated in FIG. 7 and Table 1.

[138] (Comparative Example 2)

[139] a. Synthesis of Zinc Oxide (ZnO)-Copper Oxide (Cu ₂ 0) Catalyst Particle

[140] A zinc precursor solution was prepared by dissolving 0.105 g (0.28 mmol) of a zinc precursor (Zn(acac) ₂ .6H ₂ 0; Zinc(II) acetylacetonate hydrate) and 1 g (9 mmol) of polyvinyl pyrrolidone (PVP) in 40 ml of pentanediol (PD), and the zinc precursor solution was slowly heated to 250°C for 10 minutes under inert conditions. After the zinc precursor solution was stirred at 250 °C for 3 minutes, a copper precursor solution obtained by dissolving 0.1575 g (0.6 mmol) of a copper precursor (Cu(acac) ₂ ;

Copper(II) acetylacetonate) in 5 ml of pentanediol was added thereto. Thereafter, a reaction was carried out at the same temperature for 30 minutes, and the reactant was cooled to room temperature (23 °C). The reactant was washed several times with ethanol and acetone, thereby preparing a zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle (average particle size: 40 nm, yield = 15 %), and a particle structure, a hybrid structure, and a specific surface area of the catalyst particle were measured by the method of Example 1.

[141] As a result, it may be appreciated that the catalyst particle having a hybrid structure of zinc oxide (ZnO) and copper oxide (Cu ₂ 0), which had a core-shell structure, was formed.

[142] Further, a core diameter D _p of the catalyst particle, a characteristic length L _c of

copper oxide, and a ratio of a surface area of an exposed zinc oxide core that was not treated with copper oxide to an entire surface area of the catalyst particle were illustrated in the following Table 1.

[143] b. Reduction Reaction of Carbon Dioxide Using Catalyst Particle

[144] Further, a reduction reaction was performed by the same method as in Example 1 except for using the zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle prepared by the preparation method as described above instead of the zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle of Example 1 in the reduction reaction of carbon dioxide using the catalyst particle of Example 1, and a conversion rate to methane and a methane production amount were illustrated in Table 1.

[145] (Comparative Example 3)

[146] After a zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle (average particle size:

40 nm, yield = 15 %) was prepared by the same method as in Comparative Example 2 except for using 0.0525 g (0.2 mmol) of copper oxide (Cu ₂ 0) in synthesizing the zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle of Comparative Example 2, a particle structure, a hybrid structure, and a specific surface area of the catalyst particle were measured by the method of Comparative Example 3. The results were illustrated in the following Table 1.

[147] Further, a reduction reaction was performed by the same method as in Comparative Example 2 except for using the zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle prepared by the preparation method as described above instead of the zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle prepared by the preparation method of Comparative Example 2 in the reduction reaction of carbon dioxide using the catalyst particle of Comparative Example 2, and a conversion rate to methane and a methane production amount were illustrated in Table 1.

[148] (Comparative Example 4)

[149] a. Synthesis of Zinc Oxide (ZnO)-Copper Oxide (CuO) Catalyst Particle

[150] [123] In synthesizing the zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle of Example 1 , the prepared zinc oxide (ZnO)-copper oxide (Cu ₂ 0) was dispersed in 25 ml of ethanol, and then 1 ml of I M sodium hydroxide was added thereto. Thereafter, the mixture was stirred at room temperature for 1 hour, the reactant was washed several times with ethanol and acetone, thereby preparing a zinc oxide (ZnO)-copper oxide (CuO) catalyst particle having a core-branch structure.

[151] b. Reduction Reaction of Carbon Dioxide Using Catalyst Particle

[152] Further, a reduction reaction was performed by the same method as in Comparative Example 3 except for using the zinc oxide (ZnO)-copper oxide (CuO) catalyst particle having a core-branch structure, prepared by the preparation method as described above instead of the zinc oxide (ZnO)-copper oxide (Cu ₂ 0) catalyst particle prepared by the preparation method of Comparative Example 3 in the reduction reaction of carbon dioxide using the catalyst particle of Comparative Example 3, and a conversion rate to methane and a methane production amount were illustrated in Table 1. [153] [Table 1 ]

[154] As illustrated in Table 1, it may be appreciated that the catalyst particle according to the present invention, satisfying specific parameters had excellent photocatalytic activity, such that conversion efficiency to methane by the reduction reaction of carbon dioxide was excellent, and the methane production amount of the catalyst particle was high (see FIG. 5).