Patent Application
20230226529
The present application claims priority to Korean Patent Application No. 10-2021-0125602, filed Sep. 23, 2021, the entire contents of which is incorporated herein for all purposes by this reference.
The present invention relates to an electrode including a metal oxide-based electrode and a metal phosphate thin film layer disposed on a surface thereof, and more particularly to, a metal oxide-based electrode catalyst with high efficiency in decomposition reaction of ammonia within an aqueous solution.
In order to reuse water resources, there is a growing demand for technologies that can efficiently remove ammonia in wastewater, which is a persistent pollutant, but existing ammonia removal methods, such as air stripping, breakpoint chlorination, ion exchange, biological nitrification-denitrification, and ozone treatment, generally have low ammonia removal efficiency.
Conventionally, according to a biological nitrogen removal method (biological nitrification-denitrification), when the concentration of ammonia in wastewater is high, aerobic and anaerobic activity of bacteria is suppressed, and the air stripping, which is a method of converting ammonium ions in wastewater into gaseous ammonia, is not economical due to low efficiency and large power consumption in winter.
Accordingly, a water treatment method using an electrochemical ammonia oxidation reaction of a similar process to oxidation reaction of water (H2O) has been attracting attention. However, in a decomposition reaction process by the ammonia oxidation reaction, contrary to the expectation that the ammonia oxidation reaction will show high energy conversion efficiency due to a low oxidation potential value, the conventional electrochemical method has a limitation in that it requires a high polarization potential in order to obtain a stable response current.
In addition, when a general metal electrode is used as an electrode for a decomposition reaction, since the general metal electrode does not have a band gap, which is a characteristic of a semiconductor, an oxidation reaction occurs depending on an external power, and accordingly, there is a limit in that a high overpotential is required.
Therefore, there is a need for a method for decomposing ammonia in wastewater, which increases the efficiency by inducing a decomposition reaction by selective oxidation reaction for ammonia and at the same time improves economic efficiency by reducing power to be applied.
(Patent Document 1) KR Patent Publication No. 10-2019-0122036A
Therefore, the present invention has been made to address the above problems, and it is an object of the present invention to provide a photoelectrochemical and electrochemical electrode catalyst that induces a photoelectrochemical oxidation reaction capable of efficiently removing ammonia in wastewater.
The technical object to be achieved by the present invention is not limited to the above-described technical object, and other technical objects that are not mentioned will be clearly understood by those of ordinary skilled in the art from the following description.
To accomplish the above object, according to one aspect of the present invention, there is provided a method of preparing a photoelectrochemical and electrochemical electrode catalyst, the method including: preparing a metal oxide-based electrode; introducing a phosphate layer on a surface of the metal oxide-based electrode; and converting the phosphate layer into an oxyhydroxide layer by performing electrochemical activation on the phosphate layer.
In an embodiment of the present invention, the metal oxide may include iron oxide.
In addition, the iron oxide may have a rhombohedral structure.
In addition, in an embodiment of the present invention, the oxyhydroxide layer may have a thickness of 1 nm to 10 nm.
In addition, in an embodiment of the present invention, the phosphate layer may include an iron phosphate layer.
In addition, in an embodiment of the present invention, the phosphate layer may include an amorphous structure.
In addition, in an embodiment of the present invention, the oxyhydroxide layer may include iron oxyhydroxide.
In addition, in an embodiment of the present invention, the oxyhydroxide layer may include a disordered amorphous structure.
To accomplish the above technical object, according to another aspect of the present invention, there is provided a photoelectrochemical and electrochemical electrode catalyst including: a metal oxide-based electrode; and an oxyhydroxide layer disposed on a surface of the electrode.
In an embodiment of the present invention, the oxyhydroxide layer may have a thickness of 1 nm to 10 nm.
In addition, in an embodiment of the present invention, the metal oxide-based electrode may include iron oxide.
In addition, the iron oxide may have a rhombohedral structure.
In addition, in an embodiment of the present invention, the oxyhydroxide layer may include iron oxyhydroxide.
In addition, in an embodiment of the present invention, the oxyhydroxide layer may include a disordered amorphous structure.
In the following description, the present invention is described with reference to the accompanying drawings. However, the present invention may be implemented in various forms, and thus, is not limited to embodiments described herein. In addition, irrelevant descriptions are omitted to clearly explain the present invention, and throughout the specification, the same or corresponding elements are indicated by the same reference numerals.
Throughout the specification, when a portion is connected (accessed, contacted, or coupled) with other portions, it includes direction connection as well as indirect connection in which the other member is positioned therebetween. Furthermore, throughout the specification, when a portion “includes” an element, another element may be further included, rather than excluding the existence of the other element, unless otherwise described.
The terms used in the present specification are merely used to describe particular embodiments, and are not intended to limit the inventive concept. The expression of singularity in the specification includes the expression of plurality unless clearly specified otherwise in context. In the present specification, it is to be understood that the terms such as “including,” “having,” and “comprising” are intended to indicate the existence of the features, numbers, steps, actions, elements, parts, or combinations thereof disclosed in the specification, and are not intended to preclude the possibility that one or more other features, numbers, steps, actions, elements, parts, or combinations thereof may exist or may be added.
The expression “B/A-based electrode” used in the present specification is to be understood as a form in which a B material is deposited on an A-based electrode.
To solve the shortcomings of the related art, in an embodiment of the present invention, there is provided a photoelectrochemical and electrochemical electrode catalyst including a metal oxide-based electrode and an oxyhydroxide layer positioned on a surface of the electrode.
At this time, the oxyhydroxide layer is formed through a step of introducing an amorphous phosphate layer, and the oxyhydroide layer is realized to have a disordered amorphous structure on the surface of the electrode through the amorphous phosphate layer, and increases the selectivity of ammonia oxidation reaction through the disordered amorphous structure.
An ammonia oxidation reaction process is described using a photoelectrochemical and electrochemical electrode catalyst provided by an embodiment of the present invention.
As shown in
In an embodiment of the present invention, the metal oxide-based electrode may include a transition metal oxide-based electrode and a metal oxide-based electrode including iron (Fe) may be used. However, the present invention is not limited to iron (Fe), and any known metal used for a transition metal oxide may be used without limitation.
In addition, iron oxide (α-Fe2O3) is not toxic, consists of elements abundant in the earth's crust, and has high photochemical stability, a narrow band gap (1.9 eV to 2.2 eV), and an appropriate electronic band structure, and thus, is a suitable material as a photoelectrochemical water-splitting electrode, and thus, the metal oxide-based electrode including iron may be an electrode including iron oxide, and desirably, the iron oxide may include iron oxide having a rhombohedral structure. However, the present invention is not limited to the iron oxide and not limited to the rhombohedral structure. In addition, in consideration of the toxicity of the material, the electrical characteristics of the material, and the economic efficiency of the material, a transition metal oxide compound for providing an effect of the present invention should be interpreted as being included in the scope of the present invention.
A method of preparing a photoelectrochemical and electrochemical electrode catalyst provided by an embodiment of the present invention may include preparing a metal oxide-based electrode 100, introducing an amorphous phosphate layer 200 on a surface of the electrode (step S100), and converting the amorphous phosphate layer into an oxyhydroxide layer 300 (step S200).
The oxyhydroxide layer produced through the amorphous phosphate layer is realized to have a disordered amorphous structure on the surface of the electrode, and increases the selectivity of ammonia oxidation reaction through the disordered amorphous structure.
In addition, in the introducing of the amorphous phosphate layer on the surface of the metal oxide-based electrode (step S100), a chemical vapor deposition (CVD) process may be used, and the amorphous phosphate layer may have a thickness of 1 nm to 10 nm.
In addition, in the CVD process, the thickness and uniformity of the amorphous phosphate layer may be controlled according to conditions such as the amount of precursor, reaction temperature, reaction pressure, and reaction time, and accordingly, catalytic characteristics and performance of the photoelectrochemical and electrochemical electrode catalyst which is a final product may be controlled.
In addition, in the converting of the amorphous phosphate layer into the oxyhydroxide layer (step S200), an electrochemical activation process may be used. The phosphate layer may have an amorphous structure by reacting with a hydroxide ion in an electrolyte through the electrochemical activation process.
In addition, catalytic characteristics and performance of the photoelectrochemical and electrochemical electrode catalyst may also be controlled by controlling an applied potential window in the electrochemical activation process, the number of repeated executions, the type of the electrolyte, and the like.
In addition, the phosphate layer may include an amorphous structure, and the oxyhydroxide layer may include a disordered amorphous structure.
Hereinafter, an embodiment of the present invention is described in detail with reference to the accompanying drawings. However, the present invention is not limited to the following Examples and Experimental Examples.
In Preparation Example 1, a photoelectrochemical and electrochemical electrode catalyst using a rhombohedral iron oxide (Fe2O3) (α-Fe2O3)-based electrode was prepared.
In Preparation Example 1, preparing an iron oxide based electrode through hydrothermal synthesis, preparing a FePi/Fe2O3-based electrode by uniformly depositing an amorphous iron phosphate layer (FePi layer) on a surface of the prepared iron oxide-based electrode through a CVD process, and converting a phosphate layer of the prepared FePi/Fe2O3-based electrode into a form of iron oxyhydroxide (FeOOH) through an electrochemical activation process were carried out, thereby preparing the photoelectrochemical and electrochemical electrode catalyst of A-FePi/Fe2O3. A detailed preparing process is as follows.
In Preparation Example 1.1, an iron oxide-based electrode was prepared through hydrothermal synthesis. A detailed preparation process is as follows.
1) Prepare 30 ml of aqueous solution containing 1.35 g of iron chloride hexahydrate (FeCl3·6H2O) and 0.9 g of urea.
2) Perform hydrothermal synthesis for six hours at 110° C. in an electrical oven after putting the above aqueous solution into a Teflon autoclave reactor with a capacity of 50 ml.
3) Perform heat treatment for two hours at 550° C. in an electric furnace, followed by heat treatment for 20 minutes at 800° C.
Through the above process, an α-Fe2O3-based electrode was finally synthesized.
In Preparation Example 1.2, an amorphous iron phosphate layer was uniformly deposited, through a CVD process, on a surface of the α-Fe2O3-based electrode synthesized through Preparation Example 1.1, thereby synthesizing a FePi/Fe2O3-based electrode. A detailed preparing process is as follows.
1) Place 50 mg of sodium hypophosphite monohydrate (NaH2PO2·H2O) to be apart from the α-Fe2O3-based electrode by a certain distance while appropriately apart from a gas outlet within a deposition tube.
2) Next, perform a heat treatment process at 300° C. for two hours in an argon (Ar) atmosphere.
Through the above process, finally, the amorphous iron phosphate layer was uniformly deposited on the surface of the α-Fe2O3-based electrode, thereby synthesizing a FePi/Fe2O3-based electrode.
In Preparation Example 1.3, the iron phosphate layer which is an amorphous phosphate layer deposited through Preparation Example 1.2 was converted into a form of an iron oxyhydroxide (FeOOH) layer through an electrochemical activation process.
As a detailed preparing method, cyclic voltammetry (CV) was repeated several times using, as an electrolyte, 1 M aqueous sodium hydroxide (NaOH) solution having a pH of 14. In addition, the above process was carried out in a three-electrode system, wherein the FePi/Fe2O2-based electrode synthesized in Preparation Example 1.1 and 1.2 was used as a working electrode, platinum (Pt) was used as a counter electrode, and an Hg/HgO electrode for alkaline solution was used as a reference electrode.
Through the above process, an activated FePi/Fe2O3 photoelectrochemical and electrochemical electrode catalyst (A-FePi/Fe2O3) was prepared.
In Experimental Example 1, in order to check whether the FePi layer was uniformly deposited, through the CVD process, on the surface of the α-Fe2O3-based electrode to a thickness of several nanometers in the photoelectrochemdcai and electrochemical electrode catalyst including the FePi/α-Fe-based electrode (A-FePi/Fe2O3) prepared in Preparation Example 1, analysis was performed by using a field emission scanning electron microscope (FESEM), a high-resolution transmission electron microscope (HRTEM), and an energy dispersive spectroscopy (EDS) elemental mapping through a high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM).
As shown in (a) of
In addition, as shown in (b) and (c) of
In addition, as shown in (d) of
In Experimental Example 2, a change in the binding state between elements on the surface of the electrode before and after the introduction of a metal phosphate thin film layer was analyzed.
As shown in
In Experimental Example 3, changes in the bonding state between elements on the surface of the electrode in response to the FePi phosphate layer, which was introduced on the surface of the α-Fe2O3-based electrode, undergoing electrochemical activation were measured.
As shown in
In Experimental Example 4, in order to identify that photoelectrochemical catalytic characteristics of the A-FePi/Fe2O3 photoelectrochemical and electrochemical electrode catalyst synthesized in Preparation Example 1 is selective for ammonia oxidation reaction compared to competitive water oxidation reaction, a photoelectrochemical response current according to the presence or absence of ammonia in the electrolyte was identified by cyclic voltammetry.
Hereinafter, descriptions are made with reference to
In Experimental Example 4, variation of charge transfer resistance Rct was identified by deriving Nyquist plot through EIS in order to identify improvement of electrode catalyst surface reaction performance. 0.1 M aqueous sodium hydroxide solution was used as a reference electrolyte, and accordingly, catalytic characteristics for water oxidation reaction could be identified.
In order to identify catalytic characteristics for ammonia oxidation reaction, 0.1 M aqueous sodium hydroxide solution in which concentration of ammonia ions was 550 ppm by adding ammonium solution to the reference electrolyte was used. In a three-electrode system, the A-FePi/Fe2O3 photoelectrochemical and electrochemical electrode catalyst was used as a working electrode, platinum (Pt wire) was used as a counter electrode, and a Hg/HgO electrode for alkaline solution was used as a reference electrode.
As a result of cyclic voltammetry experiment, when light was irradiated onto the A-FePi/Fe2O2 photoelectrochemical and electrochemical electrode catalyst, the electrolyte to which ammonia was added had a higher current density than that of the reference electrolyte, and accordingly, characteristics of the electrode catalyst as a photoelectrochemical ammonia oxidation reaction catalyst were identified.
In addition, as a result of the EIS experiment, it was identified through the Nyquist plot that in the case of the Fe2O3-based electrode, there was no difference in open forms of semicircles depending on the electrolyte, whereas in the case of the A-FePi/Fe2O photoelectrochemical and electrochemical electrode catalyst, semicircles became larger in the electrolyte to which ammonia was added. Through the increased Rct value of the A-FePi/Fe2O3 photoelectrochemical and electrochemical electrode catalyst, it was identified that catalytic characteristics were improved so that ammonia could be adsorbed competitively with water on the surface of the electrode by applying the technique of the present invention.
In Experimental Example 5, the degree of decomposition over time was measured using UV-Visible spectroscopy (UV-VIS) equipment in order to identify photo-catalytic (PC), electro-catalytic (EC), and photoelectrochemical-catalytic (PEC) ammonia oxidation performance of the A-FePi/Fe2O3 photoelectrochemical and electrochemical electrode catalyst.
Hereinafter, Experimental Example 5 is described with reference to
In Experimental Example 5, the experiment was conducted to identify a reaction rate order and reaction coefficient of the electrode with respect to PEC, PC, and EC ammonia oxidation reaction, and as shown in
In Experimental Example 6, an experiment was conducted to evaluate stability and performance of the A-FePi/Fe2O3 photoelectrochemical and electrochemical electrode catalyst against ammonia oxidation for a long period of time.
As shown in
In Experimental Example 7, the changed bonding state between elements on the surface of the electrode after the ammonia oxidation reaction was analyzed.
As shown in
In addition, through O 1s spectrum (b) and P 2p spectrum (c), it was identified that the iron phosphate was continuously converted into iron oxyhydroxide during the ammonia oxidation reaction process. Accordingly, it could be identified that the surface of the electrode continued to provide an active site to which ammonia can be adsorbed, thereby allowing the reaction to continue for a long period of time.
In Comparative Example 1, the catalytic activity for ammonia oxidation reaction of the electrode without electrochemical activation after the introduction of the metal phosphate thin film layer was identified.
In Comparative Example 1, the FePi/Fe2O3 electrode without the electrochemical activation process was synthesized to identify the necessity of the electrochemical activation process, which is the purpose of this experiment, and a photoelectrochemical response current according to the presence or absence of ammonia within the electrolyte was identified by cyclic voltammetry.
As a result of the experiment, when light was irradiated, the response current appeared similar regardless of the presence or absence of ammonia within the electrolyte, and thus, it was identified that there was no selective catalytic characteristics for the ammonia oxidation reaction.
In other words, it was identified that the electrochemical activation process was a necessary step to exhibit characteristics of the ammonia oxidation catalyst in combination with the introduction of the amorphous phosphate layer.
In Comparative Example 2, the catalytic activity for ammonia oxidation reaction after electrochemical activation of the electrode without the metal phosphate thin film layer was identified in order to identify the necessity of the introduction of phosphate, which is the purpose of this experiment.
As shown in
In other words, it was identified that the introduction of the amorphous phosphate layer was a necessary step to express characteristics of the ammonia oxidation catalyst in combination with the electrochemical activation process.
According to an embodiment of the present invention, when a transition metal oxide-based electrode absorbs sunlight, an electron-hole pair is formed, and an effect of exhibiting characteristics suitable for water (H2O) oxidation reaction may be provided.
In addition, according to an embodiment of the present invention, an oxyhydroxide layer may provide an effect of tuning the catalytic characteristics of a metal oxide-based electrode suitable for water (H2O) oxidation reaction to have selective characteristics for ammonia (NH3) oxidation reaction.
The effects of the present invention are not limited to the above-described effects, and it should be understood that the effects include all effects that can be inferred from the configuration of the invention described in the detailed description of the invention or the claims.
The above description of the present invention is for illustration, and those of ordinary skill in the art to which the present invention pertains can understand that it can be easily modified into other specific forms without changing the technical spirit or essential features of the present invention. Therefore, it should be understood that the embodiments described above are illustrative in all respects and not restrictive. For example, each element described as a single type may be implemented in a distributed form, and likewise elements described as distributed may be implemented in a combined form.
The scope of the present invention is indicated by the following claims, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be construed as being included in the scope of the present invention.
100: metal oxide-based electrode
200: amorphous phosphate layer
300: oxyhydroxide layer
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2021-0125602 | Sep 2021 | KR | national |
