KR20200095601A - Noble Metal Nano-star and the Catalyst Containing the Same - Google Patents

Abstract

A nano-star according to the present invention satisfies: I) a diameter of 60 nm or less; II) a noble metal having a face centered cubic (FCC) crystal structure; and III) a facet where a high index crystal plane forms a surface in which at least one of {h, k, l} of a crystal plane family defined by the Miller index satisfies a natural number of 3 or higher.

Description

Noble Metal Nano-star and the Catalyst Containing the Same}

The present invention relates to a noble metal nanostar and a catalyst comprising the same, and more particularly, to a noble metal nanostar having a controlled surface, a catalyst comprising the same, an electrode, a carbon dioxide reduction method and apparatus.

The electrochemical reduction reaction of carbon dioxide is a method of converting carbon dioxide, a greenhouse gas, into valuable compounds through electrical energy. This can reduce carbon dioxide, which is a greenhouse gas in the atmosphere, and can store electrical energy as chemical energy, thus enabling various applications in the chemical and energy industries. In particular, carbon monoxide synthesized from the reduction reaction of carbon dioxide is a synthetic gas and is widely used in the chemical industry.

As a catalyst for producing carbon monoxide from carbon dioxide, precious metals such as gold or silver having high electrical conductivity and suitable carbon monoxide binding energy are mainly used (Korean Patent Laid-Open No. 2018-0088195). However, in the case of noble metal catalysts, studies are being conducted to reduce the content of noble metals in the catalyst while maintaining catalyst efficiency due to the cost of the expensive catalyst.

In addition, research has been conducted to minimize the use of noble metals by reducing the content of noble metals in the catalyst or developing materials that replace noble metals, and by nanostructured noble metal catalysts to improve activity and selectivity.

When the size of the noble metal particles is reduced, the number of atoms exposed on the surface increases, and the surface-to-volume ratio of the particles is increased, so that a high catalyst efficiency relative to the mass can be obtained. In addition, it is known that the nanostructured catalyst has a great influence on the activity and selectivity of the catalyst during carbon dioxide reduction.

Accordingly, as the size of the particles increases, the surface-to-volume ratio of the particles decreases. Therefore, there is a need to develop a noble metal nanostructure technology having a controlled structure capable of exhibiting improved catalytic activity and selectivity as small as possible.

Republic of Korea Patent Publication No. 2018-0088195

It is an object of the present invention to provide a noble metal nanostructure having a controlled surface.

Another object of the present invention is to provide a noble metal nanostructure having excellent catalytic ability and long-term durability when reducing carbon dioxide.

Another object of the present invention is to provide a carbon dioxide reduction catalyst, an electrode including a carbon dioxide reduction catalyst, and a carbon dioxide reduction method and apparatus using the same.

The noble metal nanostar according to the present invention satisfies the following I), II), and III).

I) a diameter of 60 nm or less,

II) Noble metal with face centered cubic (FCC) crystal structure,

III) A high index crystal plane in which at least one of the {h, k, l} crystal plane family criteria defined by the Miller index, h, k, and l satisfies a natural number of 3 or more A facet that is a forming area.

The noble metal nanostar according to an embodiment of the present invention may further satisfy the following IV).

IV) Polygonal cone-shaped irregularities including the facets.

In the noble metal nanostar according to an embodiment of the present invention, the surface of the noble metal nanostar includes a facet, which is a planar surface area by the high exponential surface, and a smoothly curved surface area connecting facets adjacent to each other; I can.

In the noble metal nanostar according to an embodiment of the present invention, the total surface area of the noble metal nanostar may be 100, and the facet occupied 70% or more.

In the noble metal nanostar according to an embodiment of the present invention, the surface of the noble metal nanostar may be formed of irregularities in the shape of a polygonal cone in which the facets are positioned on each side of the polygonal horn.

In the noble metal nanostar according to an embodiment of the present invention, the {h, k, l} is {3, 2, 1}, {4, 2, 1}, {4, 3, 1}, {4, 3, 2}, {6, 4, 3}, {6, 3, 1}, {5, 4, 3}, {7, 2, 1}, {7, 4, 2}, {7, 5, It may be one or more selected from 3} and {7, 6, 1}.

In the noble metal nanostar according to an embodiment of the present invention, the noble metal may be gold, silver, or an alloy thereof.

The noble metal nanostar according to an embodiment of the present invention may be a catalyst for reducing carbon dioxide.

The present invention includes a catalyst for reducing carbon dioxide comprising the above-described noble metal nanostars.

In the catalyst for reducing carbon dioxide according to an embodiment of the present invention, the reduction may be electrochemical reduction.

The catalyst for carbon dioxide reduction according to an embodiment of the present invention has a mass activity of 33A/g or more per unit mass of a metal nanostar under a condition of -0.6V compared to a reversible hydrogen electrode (RHE). I can.

The catalyst for carbon dioxide reduction according to an embodiment of the present invention may have a CO Faraday efficiency of 80% or more when continuously reducing carbon dioxide under a condition of -0.7V for 5 hours compared to a reversible hydrogen electrode (RHE).

The catalyst for carbon dioxide reduction according to an embodiment of the present invention may further include a support on which the noble metal nanostar is supported.

The present invention includes an electrode for reducing carbon dioxide comprising the catalyst described above.

The present invention includes a carbon dioxide conversion method in which a working electrode and a fluid containing carbon dioxide are brought into contact with the above-described carbon dioxide reduction electrode as a working electrode, and a voltage is applied between the working electrode and the counter electrode of the working electrode to generate carbon monoxide.

The present invention is a working electrode that is the above-described carbon dioxide reduction electrode; The counter electrode of the working electrode; A separator (membrane) positioned between the working electrode and the counter electrode; And a carbon dioxide reduction device including a carbon dioxide-containing electrolyte.

The precious metal nanostars according to the present invention are characterized by having a very small size and a well-defined surface crystallographically located on the surface of the nanostars.

When used as a catalyst for the reduction of carbon dioxide, the noble metal nanostar according to the present invention has very excellent activity and carbon monoxide selectivity, and at the same time has a very large mass activity, so that the amount of noble metal can be significantly reduced.

1 is a transmission electron microscope photograph of a gold nanoseed (FIG. 1(a)) and a gold nano-star (FIG. 1(b)) prepared in the Example before being overgrown in Example.
FIG. 2 is a diagram showing a three-dimensional modeled nano-star (FIG. 2(b)) based on a high-magnification transmission electron microscope observation photograph (FIG. 2(a)) and a transmission electron microscope analysis result of the prepared gold nanostar.
3 is a diagram showing a partial current density of CO according to a voltage (vs. RHE).
4 is a diagram showing mass activity according to voltage (vs. RHE).
5 is a diagram showing the CO Faraday efficiency according to the voltage (vs. RHE).
6 is a diagram showing the Faraday efficiency of gas generated during a carbon dioxide reduction reaction.
7 is a diagram showing specific activities according to voltage (vs. RHE).
8 is a diagram showing the durability (durability) test results of Au nanostars and Au icosahedron during a carbon dioxide reduction reaction.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. The drawings introduced below are provided as examples in order to sufficiently convey the spirit of the present invention to those skilled in the art. Accordingly, the present invention is not limited to the drawings presented below, but may be embodied in other forms, and the drawings presented below may be exaggerated to clarify the spirit of the present invention. At this time, unless there are other definitions in the technical terms and scientific terms used, they have the meanings commonly understood by those of ordinary skill in the art to which this invention belongs, and the gist of the present invention in the following description and accompanying drawings Descriptions of known functions and configurations that may be unnecessarily obscure will be omitted.

The noble metal nanostar according to the present invention satisfies I), II), and III).

I) diameter less than 60nm

II) Noble metal with face centered cubic (FCC) crystal structure

III) A high index crystal plane in which at least one of the {h, k, l} crystal plane family criteria defined by the Miller index, h, k, and l satisfies a natural number of 3 or more Facet, the area to be formed

The nanostar may mean a shape including one or more, specifically two or more, more specifically 3 to 30, and even more specifically 8 to 20 protrusions protruding from a single center and axially reduced in the protruding direction. Two or more protrusions formed on the nanostar may have a symmetrical or asymmetrical relationship, and in one embodiment, may have a symmetrical relationship.

The diameter may mean a diameter converted into a circle having the same area as the projection image based on the projection image of the precious metal nanostar. Experimentally, the projected image may mean an image on a transmission electron microscope observation photograph of a noble metal nanostar. The average diameter may mean a value obtained by an average of diameters of at least 50 or more precious metal nanostars.

As the precious metal nanostars having an average diameter of 60 nm or less have an improved specific surface area, they are advantageous for use as a catalyst mainly subject to a surface reaction, and in particular, are advantageously sized to be supported on a support. In one embodiment of the present invention, the average diameter of the nanostars may be 60 nm or less, specifically, 20 to 60 nm, 30 to 55 nm, or 40 to 55 nm.

Precious metals having an FCC structure may include gold (Au), silver (Ag), platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), or an alloy thereof. However, when considering utilization as a catalyst for the carbon dioxide reduction reaction, the noble metal having an FCC structure is preferably gold, silver, or an alloy thereof.

The facet is a high exponential surface in which at least one of h, k and l satisfies a natural number of 3 or more, based on the {h, k, l} crystal plane family standard defined by the Miller index, on the surface of a precious metal nanostar. An index crystal plane) may mean a surface area forming a surface. At this time, as the size of the crystal lattice reference crystal is substantially infinite, the upper limits of h, k, and l are meaningless, but in view of the nano-dimension of the precious metal nanostar, at least one of h, k and l is 3 to 50 , As a specific example, it may be a natural number of 3 to 10. In the following description,'{h, k, l} plane' means one or more crystal planes belonging to the {h, k, l} crystal plane family. Miller's index is an index representing a crystal plane in the crystal structure of Bravais lattices.Based on the reciprocal lattice vectors b1, b2, and b3, hb1 + kb2 + lb3 (h of the inverse lattice vector, k and l refer to a plane that is independent of each other and perpendicular to an integer). The crystal plane by the Miller index, the family of crystal planes are well known to those skilled in the field of materials (see Neil W. Ashcroft and N. David Mermin, Solid State Physics (Harcourt: New York, 1976), etc.)

Shapely, the facet may mean a flat surface area. The length of one side of the facet may be 3 nm or more, more specifically, 5 to 15 nm. When observing the noble metal nanostar with a transmission electron microscope, the noble metal nanostar has a very fine size of 60 nm or less, so that it can be observed in the form of a projection image in which a shadow is formed by the uneven(s). Thus, experimentally, the facet has a straight line, specifically, a straight line of 2 nm or more, in a transmission electron microscope (including HRTEM) observation photo, of the noble metal nano-star image corresponding to the surface of the noble metal nano-star. Specifically, an area observed as a straight line of 3 to 15 nm can be interpreted as a surface area by a facet.

In the present invention, the {h, k, l} crystal plane family criterion defined by the Miller index, h, k and l, respectively, is a low index crsytal plane that satisfies an integer less than or equal to 2 based on the absolute value It should not be construed as excluding areas forming this surface.

However, in an advantageous example of the present invention, the noble metal nanostar may not include a planar surface area caused by a low water surface.

In one embodiment of the present invention, the surface of the noble metal nanostar includes a facet, which is a planar surface area by a high exponential surface, and a smoothly curved surface area (hereinafter, referred to as a rounding area) connecting facets adjacent to each other; In a practical example, the surface of the noble metal nanostar may be formed of a facet and a rounding region. In this case, the smoothly curved region (rounding region) refers to a region in which the slope of the tangent line continuously changes on the projection image or cross-sectional image of the noble metal nanostar.

In one embodiment of the present invention, the surface of the noble metal nanostar may include a facet that is a planar surface area due to a high exponential surface, and the total surface area of the metal nanostar is 100, and the surface area occupied by the facet is 70% or more. , Specifically 70 to 90%, more specifically, it may be 72 to 85%.

In one embodiment of the present invention, the surface of the noble metal nanostar may include a facet, which is a surface area of the surface, and a smoothly curved surface area (rounding area) connecting facets adjacent to each other. The total surface area is 100, and the surface area occupied by the facets may be 70% or more, specifically 70 to 90%, and more specifically, 72 to 85%.

The facet's crystal plane is a high index crystal plane in which at least one of the {h, k, l} crystal plane family criteria defined by the Miller index, h, k, and l satisfies a natural number of 3 or more. , As a specific example, the high exponential surface is {h, k, l}, {3, 2, 1}, {4, 2, 1}, {4, 3, 1}, {4, 3, 2}, {6 , 4, 3}, {6, 3, 1}, {5, 4, 3}, {7, 2, 1}, {7, 4, 2}, {7, 5, 3} and {7, 6 , 1}, and when considering utilization as a catalyst for the carbon dioxide reduction reaction through previous experiments, in terms of excellent catalytic activity and carbon monoxide selectivity, the high water surface is {3, 2, 1}, {7 , 5, 3} and may be one or more selected from {7, 4, 2}, and most advantageously, it is advantageous that the high exponential surface necessarily includes the {3, 2, 1} plane.

As precious metals have FCC crystal structure, the {3, 2, 1} planes are (3, 2, 1), (3, 1, 2), (2, 3, 1), (2, 1, 3) , (1, 3, 2), (1, 2, 3), (-3, 2, 1), (-3, 1, 2), (-2, 3, 1), (-2, 1, 3), (-1, 3, 2), (-1, 2, 3), (3, -2, 1), (3, -1, 2), (2, -3, 1), (2 , -1, 3), (1, -3, 2), (1, -2, 3), (3, 2, -1), (3, 1, -2), (2, 3, -1 ), (2, 1, -3), (1, 3, -2), (1, 2, -3), (-3, -2, 1), (-3, -1, 2), ( -2, -3, 1), (-2, -1, 3), (-1, -3, 2), (-1, -2, 3), (-3, 2, -1), ( -3, 1, -2), (-2, 3, -1), (-2, 1, -3), (-1, 3, -2), (-1, 2, -3), ( -3, -2, -1), (-3, -1, -2), (-2, -3, -1), (-2, -1, -3), (-1, -3, It may be a crystal plane selected from one or more of -2) and (-1, -2, -3).

In one advantageous embodiment according to the present invention, the surface of the noble metal nanostar may include a facet, which is a planar surface area by a high exponential surface including {3, 2, 1} planes, and the entire metal nanostar The surface area is 100, and the surface area occupied by the facets may be 70% or more, specifically 70 to 90%, and more specifically, 72 to 85%. In a more advantageous embodiment, the surface area occupied by the facets may be the surface area occupied by the plane facets of {3, 2, 1} planes.

In one advantageous embodiment, the surface of the noble metal nanostar is a facet, which is a plane surface area by a high exponential surface including {3, 2, 1} planes; and a smoothly curved surface area that connects adjacent facets ( Rounding area), and the total surface area of the metal nanostar is 100, and the surface area occupied by the facets may be 70% or more, specifically 80% or more, and substantially 80 to 95%.

In an advantageous embodiment according to the present invention, the surface of the noble metal nanostar is a facet, which is a surface area of a plane by a high exponential surface including a {3, 2, 1} plane; and a smooth curvature connecting the facets adjacent to each other It may consist of a true surface area (rounding area). At this time, the total surface area of the metal nanostar is 100, the surface area occupied by the facets may be 70% or more, specifically 70 to 90%, more specifically, 72 to 85%, and the surface area occupied by the rounding area is 30% or less. , Specifically, it may be 10 to 30%, more specifically 15 to 28%.

In an advantageous embodiment according to the present invention, the surface of the noble metal nanostar is a facet, which is a surface area by a {3, 2, 1} plane; and a smoothly curved surface area (rounding area) connecting facets adjacent to each other It may include, and the total surface area of the metal nanostar is 100, and the surface area occupied by the facet, which is the surface area by the {3, 2, 1} plane, is 70% or more, specifically 70 to 90%, more specifically, It may be 72 to 85%. Furthermore, the surface of the noble metal nanostar may include a facet, which is a surface area formed by a {3, 2, 1} plane, and a smoothly curved surface area (rounding area) connecting facets adjacent to each other.

In terms of the shape of the nanostar, the protrusion of the nanostar may have a polygonal horn shape, and the polygonal horn may include a facet, which is a planar surface area by a high exponential surface. In other words, the facet, which is the surface area of the plane by the high exponential surface of the nanostar, can form the side surface (the surface of the pyramid) of the polygonal horn as the protrusion. At this time, the protrusion in the shape of a polygonal horn means that the facet exists (position) on the side of the polygonal horn, and the shape of the polygonal horn is limited to a shape in which the edges and corners of the polygonal horn are sharply angled. Should not be interpreted. As an example, a shape in which the edge of the polygonal horn meets the side and the edge of the polygonal horn (the tip of the polygonal horn) where three or more sides meet smoothly should be interpreted as a polygonal horn. However, the present invention does not exclude a shape in which the edges and edges are sharply angled.

In one embodiment, the surface of the noble metal nanostar may be formed of irregularities in the shape of a polygonal horn in which facets are located on each side of the polygonal horn.

In an advantageous embodiment according to the present invention, the surface of the noble metal nanostar is, on each side of the polygonal horn, a facet, which is a planar surface area by a high exponential surface including {3, 2, 1} planes, is located. It can be made of horn-shaped irregularities.

In one advantageous embodiment according to the present invention, the surface of the noble metal nanostar is a polygonal cone-shaped facet, which is a planar surface area by a high exponential surface, which is a {3, 2, 1} plane, on each side of the polygonal horn. It can be made of irregularities. In this case, the edge of the polygonal horn (the tip of the polygonal horn) where the side and the side of the polygonal horn meet, and the edge of the polygonal horn (the tip of the polygonal horn) where three or more sides meet may have a smoothly curved shape, which may correspond to the aforementioned rounding area.

In an advantageous embodiment according to the present invention, the noble metal nanostar has a total surface area of 100, and the surface area occupied by the facet of the {321} plane of the polygonal horn is 70% or more, specifically 70 to 90%. , More specifically, it may be 72 to 85%.

In one advantageous example, the noble metal nanostar may be used as a catalyst for carbon dioxide reduction. Although not necessarily limited to this interpretation, when the precious metal nanostar contains a facet of a high surface including {321} planes, *COOH, which is a rate-determining step (RDS), which determines the reaction rate of the carbon dioxide reduction reaction. In the adsorption step, very stable *COOH adsorption is provided by the {321} plane, and the bonding force between the {321} plane and *CO is small, so rapid desorption of CO can occur, and the hydrogen evolution reaction ( Adsorption of *H, an intermediate product that controls the rate of hydrogen evolution reaction (HER), does not occur well, so it can exhibit surprisingly excellent catalytic activity and CO selectivity when reducing carbon dioxide.

The present invention includes a method of manufacturing the noble metal nanostar described above.

The method for manufacturing precious metal nanostars according to the present invention is based on the {h, k, l} crystal face family defined by the Miller index, where h, k, and l are each a natural number of 2 or less to 0. crystal plane) and preparing a noble metal nanostar by heating a mixture of a noble metal seed having a size of 30 nm or less, a hydrophilic polymer, an alkylamine surfactant, an inorganic acid, and a noble metal precursor (overgrowth step) Includes.

In one embodiment, the hydrophilic polymer may be polyvinylpyrrolidone (PVP), polystyrene, poly(vinyl acetate), polyisobutylene, etc. It is not limited. In one embodiment, it may be polyvinylpyrrolidone. Mw of the hydrophilic polymer may be 10,000 to 100,000, but is not limited thereto.

In one embodiment, the alkylamine-based surfactant is monomethylamine, dimethylamine, trimethylamine, monoethylamine, diethylamine, triethylamine, isopropylamine, diisopropylamine, mono-n-propylamine, Di-n-propylamine, tri-n-propylamine, mono-n-butylamine, di-n-butylamine, tri-n-butylamine, mono-sec-butylamine, di-sec-butylamine, tri -sec-butylamine, mono-tert-butylamine, di-tert-butylamine, cyclohexylamine, dicyclohexylamine, octylamine, dodecylamine, hexadecylamine, dimethylamino ethylamine, N-ethylethylenediamine , N,N-dimethyl-1,3-propanediamine, or a mixture thereof. By varying the type of the alkylamine surfactant, the spherical crystal plane of the high-index surface forming the facet can be controlled. As a specific example, noble metal nanostars containing {3, 2, 1} facets may be prepared using dimethylamine, and noble metal nanostars containing {7, 5, 3} facet faces using monomethylamine Stars can be prepared, and noble metal nanostars including {7, 4, 2} facets are prepared using octylamine, etc., depending on the type of alkylamine surfactant, {3, 2, 1}, {4, 2, 1}, {4, 3, 1}, {4, 3, 2}, {6, 4, 3}, {6, 3, 1}, {5, 4, 3}, {7 , 2, 1}, {7, 4, 2}, {7, 5, 3}, and {7, 6, 1} Noble metal nanostars including facets of high water surface selected from the group or more can be prepared .

In one embodiment, the inorganic acid for controlling the reduction rate may be one or more selected from sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, sulfamic acid, perchloric acid, chromic acid, sulfurous acid, and nitrous acid, but is not limited thereto.

In one embodiment, the noble metal precursor may be a noble metal chloride, fluoride, bromide, nitrate, sulfate, acetate, halide acid, etc., based on an advantageous example of gold, such as chloroauric acid (HAuCl ₄ ) or gold chloride. However, it is not limited thereto.

In one embodiment, the solvent of the reaction solution is an ether solvent such as octyl ether, butyl ether, hexyl ether, or decyl ether, pyridine, tetrahydrofuran ( Heterocyclic compound-based solvents such as THF), aromatic compound-based solvents such as dimethyl sulfoxide (DMSO) and dimethylformamide (DMF), alcohol-based solvents such as octyl alcohol and decanol, or a mixed solvent thereof. have.

In one embodiment, the reaction solution is based on 0.1 parts by weight of each noble metal nano, 5 to 30 parts by weight of an alkylamine-based surfactant, 1 to 10 parts by weight of an inorganic acid, 100 to 300 parts by weight of a hydrophilic polymer, and 0.01 to 0.50 parts by weight of a noble metal precursor It may contain. At this time, the concentration of substances satisfying the weight ratio described above in the reaction solution may be within the solubility range of the solvent.

The heating temperature of the reaction solution may be 50 to 80°C, and the reaction time may be 3 to 6 hours.

The {h, k, l} crystal plane family standard defined by the Miller index, including the surface of the low index crystal plane where h, k, and l are natural numbers of 2 or less and 0, respectively, and is 30 nm or less The noble metal seed having a size of may be an icosahedral noble metal seed including the surface of the low water surface including the {111} plane. Such low-water noble metal seeds can be prepared using a conventionally known method.

For example, 1,5-pentanediol (1,5-pentanediol), 1,3-propanediol (1,3-propanediol), diethylene glycol (diethylene glycol) in a reducing solvent such as polyvinylpyrrolidone Hydrophilic polymers such as (polyvinylpyrrolidone, PVP), polystyrene, poly(vinyl acetate), polyisobutylene, etc. were added, heated to a temperature of 240 to 260°C, and heated. It can be prepared by injecting a precursor solution in which a noble metal precursor is dissolved in a reducing solvent of the same kind into a hydrophilic polymer solution.

The present invention includes a catalyst for reducing carbon dioxide comprising the above-described noble metal nanostars.

Carbon dioxide reduction may be electrochemical reduction, but the present invention should not be interpreted as excluding reduction by direct reaction between gaseous carbon dioxide (carbon dioxide gas) and a catalyst.

In one embodiment, the catalyst may further include a support on which the noble metal nanostars are supported together with the noble metal nanostars described above. The support may be any support known to be used to support a catalyst metal in the field of catalysis, for example, in the field of electrochemical reduction of carbon dioxide. For example, the support may be a carbon support, and the carbon support may be carbon black, ketjen black, acetylene black, mesoporous carbon, graphite, graphite nanofiber (GNF), graphene, carbon nanotube (CNT). ; Carbon Nanotube) or a mixture thereof, etc., but the present invention is of course not limited by the specific type of the support.

In one embodiment, the catalyst may contain 1 to 25% by weight, 5 to 20% by weight, or 5 to 15% by weight of noble metal nanostars.

In one embodiment, the catalyst has a mass activity of 33A/g or more per unit mass of the precious metal nanostar under -0.6V (vs. RHE) conditions compared to a reversible hydrogen electrode (RHE). I can.

In one embodiment, the catalyst has a CO current density according to a voltage compared to a reversible hydrogen electrode in the entire range of voltage (vs. RHE) of -0.9 to -1.1V, as the voltage increases in the negative direction, the CO current density. Can increase in the negative direction. That is, the slope defined by the CO current density increase/voltage increase may have a positive value in the entire voltage range of -0.9 to -1.1V.

In one embodiment, the catalyst may have a maximum CO Faraday efficiency of 87% or more in a voltage range of -0.4 to -1.1V compared to a reversible hydrogen electrode (RHE).

In one embodiment, the catalyst may have a CO Faraday efficiency of 80% or more when continuously reducing carbon dioxide under a condition of -0.7V for 5 hours compared to a reversible hydrogen electrode (RHE).

In one embodiment, the catalyst may have a specific activity of 2.0 mA/cm ² or more under a condition of -0.7 V compared to a reversible hydrogen electrode (RHE).

The CO Faraday efficiency of the catalyst, the mass activity of the catalyst, the inactivation of the catalyst, and the CO current density according to the voltage were experimentally determined by adopting a 0.5 M KHCO ₃ aqueous solution (pH = 7.3) as an electrolyte and supplying carbon dioxide to the electrolyte. It may be a carbon dioxide reduction system using a carbon dioxide saturated electrolyte, a Pt plate as an anode, and Ag/AgCl as a reference electrode, and a carbon-gold nanoparticle supported on Ketchen Black at a level of 10 to 13% by weight of Au nanostars. The star complex may be based on a working electrode loaded on a glass carbon electrode having an effective area of 0.3 to 1.00 cm 2.

The present invention includes an electrode for reducing carbon dioxide comprising the catalyst described above.

The electrode may include a current collector and an active material layer including the catalyst described above. At this time, the active material layer may further include a polymeric binder commonly used to fix the catalyst to the current collector, and may further include known additives such as a conductive material to improve the electrical properties of the catalyst layer if necessary. Of course.

The present invention includes a carbon dioxide conversion method in which a working electrode and a fluid containing carbon dioxide are brought into contact with the above-described carbon dioxide reduction electrode as a working electrode, and a voltage between the working electrode and the counter electrode of the working electrode is applied to generate carbon monoxide. The counter electrode, which is the counter electrode of the working electrode, may be platinum, gold, glass carbon, iridium, etc., but is not limited thereto.

The carbon dioxide-containing fluid may be an electrolyte in which carbon dioxide is dissolved. At this time, it goes without saying that carbon dioxide is supplied to the electrolyte and carbon dioxide dissolved in the electrolyte may be reduced by contacting the working electrode. The electrolyte of the liquid electrolyte may be an electrolyte commonly used in electrochemical carbon dioxide reduction. In a specific embodiment, the electrolyte may be selected from the group consisting of K ₂ SO ₄ , KHCO ₃ , KCl, KOH, and combinations thereof, but is not limited thereto. The space in which the working electrode is located and the space in which the counter electrode is located may be divided by a separator (membrane), and the separator may be a hydrogen ion conductive separator, but is not limited thereto.

The present invention is a working electrode that is the above-described carbon dioxide reduction electrode; The counter electrode of the working electrode; A separator (membrane) positioned between the working electrode and the counter electrode; And a carbon dioxide reduction device including a carbon dioxide-containing electrolyte.

The carbon dioxide reduction device may have a conventionally known structure such as a batch type, a circulation type, a continuous type, a polymer electrolyte membrane type system, or a mixed type consisting of a combination thereof, and as known according to the structure of the specific carbon dioxide reduction device. Of course, a line for injection, discharge and/or circulation of carbon dioxide, product, electrolyte, etc. can be constructed.

In the carbon dioxide conversion method and the carbon dioxide reduction apparatus according to the present invention, carbon dioxide is supplied and carbon monoxide can be produced with very excellent selectivity.

Accordingly, the carbon dioxide conversion method of the present invention may correspond to a method for producing (producing) carbon monoxide, and the carbon dioxide reducing apparatus of the present invention may correspond to an apparatus for producing carbon monoxide.

Accordingly, the present invention includes a method for producing (producing) carbon monoxide and an apparatus for producing carbon monoxide.

(Example 1)

Gold nanostar manufacturing

A gold precursor solution was prepared as a 1,5-pentanediol (PD; 1,5-Pentanediol) solution in which gold (I) chloride (AuCl) was dissolved at 0.050 molar concentration (M). After heating a surfactant solution in which 0.25 g of polyvinyl pyrrolidone (PVP; Polyvinyl pyrrolidone, Mw 55000) is dissolved in 5.5 ml of 1,5-pentanediol, 1.5 ml of the prepared gold precursor solution was injected. , Stirring (350 rpm) at 250° C. for 5 minutes to prepare an icosahedral gold nanoseed. Gold nanoseeds prepared by centrifugation were recovered, washed three times with ethanol, and redispersed in dimethyl formamide to prepare a gold nanoseed dispersion.

After mixing 3.6 g of polyvinylpyrrolidone, 300 μl of dimethylamine (45 wt% in H ₂ O) and 240 μl of 2.5 molar aqueous hydrochloric acid solution in 45 ml of dimethylformamide, gold nanoseed dispersion Gold nanostars were prepared by adding 1.5 ml (10 μmol based on Au metal) and 60 μl of a dimethylformamide solution in which HAuCl ₄ was dissolved at 0.050 molar concentration (M), and stirred at 65° C. for 4 hours (350 rpm). I did. The gold nanostars prepared by centrifugation were recovered, washed twice with ethanol, and then dispersed and stored in 5 ml of ethanol.

Electrode manufacturing

15.6 mg of Ketjen black quantified through ultrasonic treatment was dispersed in 50 ml of ethylene glycol so that the prepared gold nanostars were supported on the carbon support, and gold nanostars dispersed in ethanol (based on Au metal). 2.7 mg) was added. Thereafter, the mixture was stirred at room temperature for 1 hour and filtered to recover the prepared carbon-gold nanostar composite, and dried in a vacuum oven at 60° C. for 12 hours or more. Quantification of gold nanostars was performed through inductively coupled plasma emission spectroscopy (ICP-OES). As a result of ICP-OES analysis, it was confirmed that 11% by weight of gold nanostars was supported on the carbon-gold nanostar composite.

The prepared carbon-gold nanostar complex was dispersed in isopropanol (dispersion concentration = 1 mg/ml), and then applied to a glass carbon electrode (effective area 0.50㎠) by drop-casting to free carbon 0.10g of carbon-gold nanostar composite was loaded on the effective area of the electrode. Thereafter, isopropanol was added to Nafion perfluorinated resin solution (5 wt%), diluted 20 times, and 30 μl of the diluted resin solution was drop-cast on the same surface to prepare a working electrode.

Carbon dioxide reduction system manufacturing

An airtight reaction cell with separated anode chamber and cathode chamber was used as a carbon dioxide reduction system. A working electrode made of a cathode was used, a Pt plate was used as an anode, and Ag/AgCl (in 3 M NaCl) was used as a reference electrode. Carbon dioxide was bubbled in a 0.5 M KHCO ₃ aqueous solution (pH=7.3) at 60 cc/min for 1 hour, and a carbon dioxide saturated aqueous potassium bicarbonate solution was used as an electrolyte.

The anode chamber and the cathode chamber were separated by a Nafion ^® 117 ion exchange membrane, and only the cathode side was stirred at 260 rpm. The CO ₂ supply to the carbon dioxide reduction system was fixed at 60 cc/min, and the CO ₂ supply was controlled by a mass flow controller (MFC, Line Tech M3030V). However, for accurate analysis when measuring electrochemical impedance spectroscopy (EIS), CO ₂ supply and stirring were stopped.

The out-line of the cathode chamber was connected directly to the in-line of a pulsed discharge detector gas chromatography (PDD GC, YL6500GC) allowing direct product analysis of the reaction. At this time, ultra-high purity helium gas (99.9999%) was used as a carrier gas.

(Example 2)

In Example 1, except that monomethylamine was used instead of dimethylamine, a gold nanostar, electrode, and a carbon dioxide reduction system were prepared in the same manner as in Example 1.

(Example 3)

In Example 1, except that octylamine was used instead of dimethylamine, a gold nanostar, electrode, and a carbon dioxide reduction system were prepared in the same manner as in Example 1.

All electrochemical measurements were performed with Bi-potentiostat (CH Instruments, CHI760E). Before measuring the general carbon dioxide reduction reaction, in order to ensure contact between the electrolyte and the catalyst, -1V voltage was applied for 60 seconds. LSV (linear sweep voltammetry) from -1.0V to -2.2V was performed to check the onset voltage. For the carbon dioxide reduction reaction analysis using gas chromatography, apply from -2.2V to -1.0V at an interval of 0.1V, but each voltage step (interval) was maintained for 720 seconds, and the carbon dioxide reduction reaction product at each voltage step Was analyzed.

The measured potential value was converted into a reference potential value of a reversible hydrogen electrode (RHE) using the following equation.

E(vs.RHE)=E(vs.Ag/AgCl) + 0.209V + 0.0591VxpH-iR

In the equation, E(vs.RHE) is the reference potential value of the reversible hydrogen electrode, E(vs.Ag/AgCl) is the measured potential value measured using Ag/AgCl as the reference electrode, and pH is the pH value of the electrolyte, iR is the voltage drop due to the internal resistance, R is the resistance value of the carbon dioxide reduction system measured by electrochemical impedance spectroscopy, and i is the measured current value at the applied potential value. Accordingly, the reversible hydrogen electrode reference potential (voltage) value is a value obtained by supplementing the IR-drop, which is a voltage drop caused by resistance.

In order to test the catalyst durability of the prepared gold nanostars, a fixed voltage of -0.7V (vs.RHE) was applied to the working electrode for 5 hours, and the carbon dioxide reduction reaction product was applied for 10 minutes using gas chromatography. Analyzed at intervals.

Electrochemical active surface area (ESCA) was previously reported (Liu, M.; Pang, Y.; Zhang, B.; De Luna, P.; Voznyy, O.; Xu, J.; Zheng , X.; Dinh, CT; Fan, F.; Cao, C.; de Arquer, FPG; Safaei, TS; Mepham, A.; Klinkova, A.; Kumacheva, E.; Filleter, T.; Sinton, D. .; Kelley, SO; Sargent, EH Nature 2016, 537, 382-386, Bin, X.; Sargent, EH; Kelley, SO Anal.Chem. 2010, 82, 5928-5931). In detail, a CV (cyclic voltammogram) was obtained using an electrolyte that is a 50mM H ₂ SO ₄ aqueous solution, and CV data converged after repeated scans from 0V to -1.5V (vs.Ag/AgCl) at a scan rate of 50mV/s Was used. ESCA was calculated by integrating the peak at -0.9V to -1.0V derived from the reduction of monolayer chemisorbed oxygen, and the reduction charge per unit area at the time of calculation was 448 μC/cm ² .

During transmission electron microscope (TEM) analysis, gold nanostars dispersed in ethanol were dropped onto a copper grid and dried to be used as a TEM analysis sample.

For comparison, a catalyst and a working electrode were prepared in the same manner as in the Example using icosahedral gold nanoseeds instead of gold nanostars, and carbon dioxide reduction reaction was performed in the same manner as in the Example. In addition, a carbon dioxide reduction reaction was performed in the same manner as in the Example by using the gold foil itself as a working electrode.

1 is a gold nanoseed (gold icosahedron, FIG. 1 (a), scale bar = 50 nm), which is a step before overgrowth in Example 1, and gold nano-stars prepared in Example 1 (FIG. 1 (b ), scale bar = 20nm) is observed in a transmission electron microscope. As can be seen from FIG. 1, it can be seen that monodisperse Au icosahedral seeds having an average size of 23 nm are prepared, and it can be seen that Au nanostars having a symmetrical structure are manufactured through overgrowth. The average size of the prepared Au nanostars was 52 nm.

FIG. 2 shows a three-dimensional modeled nanostar (FIG. 2(b)) based on a high-magnification transmission electron microscope observation photo (FIG. 2(a)) and a transmission electron microscope analysis result of a gold nanostar prepared in Example 1. It is a drawing.

As shown in the example shown in FIG. 2, as a result of measuring the interplanar angle of the arm, which is a protruding part of the gold nanostar manufactured in various crystal axes, the arm is formed as a {321} plane, and the surface of the gold nanostar is It can be seen that the {321} plane is formed, and it can be seen that the projection image of the three-dimensional modeled nanostar with the surface of the {321} plane substantially coincides with the image of the gold nanostar on the transmission electron microscope. Experimentally, in order to prevent an error due to the tip area of the arm, the interplanar angle of the faces constituting the arm is the tip direction of the arm from the bottom of the arm close to the center of the nano star on the HRTEM image of the nano star as shown in FIG. 2(a). By drawing an imaginary straight line along a flat surface, it may be calculated using the intersection angle between the straight lines. At this time, the projection image (for example, Fig. 2(b)) from one crystal axis of the three-dimensional modeled nanostar with the surface of the {321} plane and the plane of the plane forming the arm in each arm through observation with a transmission electron microscope If the average angle difference between angles is less than 4.5˚, it can be regarded as an experimental error, and if the average angle difference is less than 4.5˚, it can be regarded as the same shape as the three-dimensional modeled nanostar.

Through high-magnification-transmission electron microscopy (HR-TEM), gold nanostars produced for various crystal axes are observed, and interplanar (facet) angle analysis and atomic resolution TEM (Single atomic resolution TEM) analysis As a result of performing, it was confirmed that although the tip of the arm had a partially curved shape, the surfaces forming each arm of the nanostar were {3, 2, 1} crystal planes, and each arm of the nanostar was {3, 2, 1} It was confirmed that it has a polygonal cone shape made of a crystal plane. In addition, when considering the smoothly curved portion at the tip of the arm, it was confirmed that the area where the {3, 2, 1} crystal plane forms the surface of the total surface area (100%) of the precious metal nanostar is 74.5%. Similarly, as a result of analyzing the nanostars prepared in Examples 2 and 3, it was confirmed that facets were formed on the side of the arm similarly to Example 1 in both the nanostars prepared in Example 2 and Example 3, In the case of the nanostar prepared in Example 2, it was confirmed that the facets of the {7, 5, 3} crystal planes were formed, and in the case of the nanostar prepared in Example 3, the facets of the {7, 4, 2} crystal planes were formed. In addition, the nanostars prepared in Examples 2 and 3 also had a structure in which the surface of the noble metal nanostar was covered with cancer, similar to Example 1, and in the total surface area (100%) of the noble metal nanostar, {7, It was confirmed that the surface area of the high-index crystal planes including 5, 3} planes or {7, 4, 2} planes occupied more than 70%.

3 to 8 are diagrams showing test results for carbon dioxide reduction catalytic activity of gold nanostars prepared according to an embodiment of the present invention. In FIGS. 3 to 8,'Au nanostars' refers to the result of a working electrode containing gold nanostars as a catalyst according to Example 1, and'Au ico.' is an experiment for comparison and overgrowth in Example 1 It refers to the result of the working electrode containing the icosahedral gold nanoseed as a catalyst, and'Au foil' refers to the result of using the gold foil itself as the working electrode.

3 is a diagram showing the CO partial current density. In detail, FIG. 3 shows a partial current density of CO according to a reversible hydrogen electrode reference voltage (vs. RHE), and as can be seen in FIG. 3, a nanostructured Au nanoseed compared to an Au foil (Au Icosahedron) or Au nanostars have a larger CO partial current density and a lower onset voltage. In addition, looking at the CO partial current density of the Au foil, Au icosahedron, and Au nanostar, it can be seen that the CO partial current density of the Au nanostar is the largest in the entire region. In addition, in the case of Au icosahedron, the CO partial current density is saturated in the region below -1.0V (vs. RHE), but in the case of Au nanostars, it is not saturated in the region below -1.0V (vs. RHE). , It can be seen that the current density of the CO part increases with the voltage.

Mass activity is an important factor in determining the amount of noble metal used, and the higher the mass activity, the same catalytic activity can be achieved with a trace amount of noble metal. Accordingly, as the mass activity increases, the amount of noble metal used for carbon dioxide reduction can be reduced, and the mass activity is a very important index indicating the commerciality of the noble metal-based catalyst.

FIG. 4 is a diagram showing the mass activity according to the reversible hydrogen electrode reference voltage (vs. RHE). Looking at FIG. 4, it can be seen that the Au nanostars have a higher mass activity than the Au icosahedron in the entire voltage range. In the case of Au nanostars, it can be seen that it has a mass activity of 33.4A/g at -0.6V (vs. RHE) and 91.9 A/g at -0.7V (vs. RHE). have. Mass activity ranging from -0.6V (vs. RHE) to 33.4A/g is the highest activity reported to date.

5 is a diagram showing CO Faradaic efficiency according to a reversible hydrogen electrode reference voltage (vs. RHE). As can be seen in FIG. 5, it can be seen that the CO Faraday efficiency varies depending on the applied voltage, and it can be seen that the nanostar has the best CO Faraday efficiency in the entire voltage range.

The maximum CO Faraday efficiency of the Au nanostar was 87.7%, the maximum CO Faraday efficiency of the Au icosahedron was 73%, and the maximum CO Faraday efficiency of the Au foil was 10.0%. In addition, as can be seen in Figures 3 and 6, it can be seen that the CO Faraday efficiency of 87.5% and the CO partial current density of -1.1mAcm ^{-2 in} the overpotential state of only 0.530V (vs. RHE) .

In order to examine the CO selectivity, the generated gases were analyzed using gas chromatography, and a trace amount of CH ₄ was generated at a low voltage, but it was confirmed that the main products were H ₂ and CO. 6 is a diagram showing the Faraday efficiency of gas generated during the carbon dioxide reduction reaction, FIG. 6(a) is a diagram showing the total current density according to the reversible hydrogen electrode reference voltage (vs. RHE), and FIG. 6(b) ) Is a diagram showing the Faraday efficiency of each product according to the reversible hydrogen electrode reference voltage in the Au icosahedron, and FIG. 6(c) is a diagram showing the Faraday efficiency of each product according to the reversible hydrogen electrode reference voltage in the Au nanostar. to be. 6, it can be seen that the Faraday efficiency of the detected gas is 100%. This means that all products produced during the reaction were analyzed without gas leakage.

7 is a diagram showing specific activities according to the reversible hydrogen electrode reference voltage (vs. RHE). Specific activity is the CO current density divided by the electrochemically active surface area (ESCA). As a result of calculating ESCA through CV measurement using an H ₂ SO ₄ aqueous solution (electrolyte), the ESCA of the Au nanostar was 0.52 cm2, the ESCA of the Au icosahedron was 1.06 cm2, and the ESCA of the Au foil was 0.38 cm2. As the average size of the Au icosahedron is 23 nm and the average size of the Au nanostars is 52 nm, it can be seen that the average size increases and the ESCA decreases.

However, looking at the inactivity of the Au nanostars and the inactivation of the Au icosahedron in FIG. 7, it can be seen that the Au nanostars have the greatest inactivity in the entire voltage range, and the Au nanostars are the inactivation of the Au icosahedron. It can be seen that it has an inactive value that is more than twice the value. On the other hand, it can be seen that the Au icosahedron has a similar inactive value to the Au foil. Through this, it can be seen experimentally that low-index crystal planes such as the {111} plane forming the surface of the Au icosahedron only act similar to the Au polycrystalline material exposed to the surface of the Au foil.

From the results of FIG. 7, it can be seen that the {321} plane forming the surface of the Au nanostars provides an active site for a much larger amount of carbon dioxide reduction reaction than the low-water surface such as the {111} plane.

FIG. 8 is a diagram showing the results of a durability test of Au nanostars and Au icosahedron, and FIG. 8(a) is a diagram showing a voltage (-0.7V (vs. RHE)) application time during a durability test. It is a diagram showing the CO Faraday efficiency, FIG. 8(b) is a diagram showing the total current density according to the voltage application time during the durability test, and FIG. 8(c) is recovered from the electrode after the durability test. It is a transmission electron microscope photograph of the Au nanostars observed, and Figure 8 (d) shows a transmission electron microscope photograph (Figure 8 (d)) observing the Au icosahedron recovered from the electrode after the Au durability test. It is a drawing.

It can be seen that the voltage application time increases as shown in FIG. 8(b) and the current density decreases slightly in both the Au nanostars and the Au icosahedron, but in the CO Faraday efficiency of FIG. 8(a), the Au nanostars and Au icosahedron It can be seen that the liver shows a very large difference. In detail, after 5 hours of voltage application, the CO Faraday efficiency of Au icosahedron rapidly decreases from 83% to 59% initially, but in the case of Au nanostars, the CO Faraday efficiency is remarkable from 90% to 81% even after 5 hours of voltage application. It can be seen that it maintains a high value.

As shown in the observation result of the transmission electron microscope of FIGS. 8(c) and 8(d), both Au icosahedron and Au nanostars show that rounding occurs at edges and corners by durability tests. Able to know. In the case of the Au icosahedron, it can be seen that the sharp edges and corners that provide the active site of the carbon dioxide reduction reaction are rounded, and the active site decreases, resulting in a rapid decrease in CO Faraday efficiency. However, in the case of the Au nanostar, it can be seen that the high CO Faraday efficiency is stably maintained as the active sites for the carbon dioxide reduction reaction are abundantly provided by the {321} plane despite the rounded edges and corners.

Similar to the catalytic ability shown through FIGS. 3 to 8, it was confirmed that the Au nanostars prepared in Examples 2 and 3 also have superior catalytic activity and carbon monoxide selectivity than Au icosahedron.

As described above, in the present invention, it has been described by specific matters and limited embodiments and drawings, which are provided to help the overall understanding of the present invention, but the present invention is not limited to the above embodiments, and the present invention Various modifications and variations are possible from those who have ordinary knowledge in the field to which they belong.

Accordingly, the spirit of the present invention is limited to the described embodiments and should not be defined, and all things having equivalent or equivalent modifications to the claims as well as the claims to be described later fall within the scope of the spirit of the present invention. .

Claims (16)

Noble metal nanostars satisfying the following I), II), and III).
I) diameter less than 60nm
II) Noble metal with face centered cubic (FCC) crystal structure
III) A high index crystal plane in which at least one of the {h, k, l} crystal plane family criteria defined by the Miller index, h, k, and l satisfies a natural number of 3 or more Facet, the area to be formed According to claim 1,
Noble metal nanostars more satisfying the following IV).
IV) Polygonal cone-shaped irregularities including the facets According to claim 1,
The noble metal nanostars comprising: a facet that is a planar surface area by the high index surface and a smoothly curved surface area connecting facets adjacent to each other. According to claim 1,
Noble metal nanostars having a total surface area of 100 and the facet occupied by 70% or more. According to claim 2,
The surface of the noble metal nanostar is a noble metal nanostar consisting of irregularities in the shape of a polygonal horn in which the facets are located on each side of the polygonal horn. According to claim 2,
{H, k, l} is {3, 2, 1}, {4, 2, 1}, {4, 3, 1}, {4, 3, 2}, {6, 4, 3}, { One selected from 6, 3, 1}, {5, 4, 3}, {7, 2, 1}, {7, 4, 2}, {7, 5, 3} and {7, 6, 1} The ideal precious metal nanostar. According to claim 1,
The noble metal is gold, silver, or an alloy thereof. The method according to any one of claims 1 to 7,
Noble metal nanostars used as catalysts for carbon dioxide reduction. A catalyst for reducing carbon dioxide comprising the noble metal nanostar of any one of claims 1 to 7. The method of claim 9,
The reduction is an electrochemical reduction catalyst for carbon dioxide reduction. The method of claim 10,
The catalyst is a catalyst for reducing carbon dioxide having a mass activity of 33A/g or more per unit mass of a noble metal nanostar at -0.6V compared to a reversible hydrogen electrode (RHE). The method of claim 10,
The catalyst is a carbon dioxide reduction catalyst having a CO Faraday efficiency of 80% or more when continuously reducing carbon dioxide under a condition of -0.7V for 5 hours compared to a reversible hydrogen electrode (RHE). The method of claim 9,
Catalyst further comprising a support on which the noble metal nanostars are supported. An electrode for reducing carbon dioxide comprising the catalyst of claim 9. Using the carbon dioxide reduction electrode according to claim 14 as a working electrode,
A carbon dioxide conversion method in which a working electrode and a carbon dioxide-containing fluid are brought into contact, and a voltage between the working electrode and the counter electrode of the working electrode is applied to generate carbon monoxide. A working electrode as an electrode for reducing carbon dioxide according to claim 14; A counter electrode of the working electrode; A separator positioned between the working electrode and the counter electrode; And a carbon dioxide-containing electrolyte.

Images