CUWO4 HOLLOW NANOSPHERE AND METHODS OF MAKING THEREOF

Abstract

Electrochemical conversion of nitrate to ammonia is an approach to alleviate nitrate pollution in water and simultaneously generate green NH3 fuels. The practical application of this approach is challenging due to the lack of efficient electrocatalysts. The present disclosure relates generally to an electrocatalyst including a plurality of CuWO4 hollow nanospheres, which include asymmetric oxygen vacancies and adjacent Mo clusters, and methods of making and using the same. The electrocatalyst of the present disclosure is designed for efficient ammonia electrosynthesis.

Description

FIELD

The present disclosure relates generally to electrocatalysts including CuWO₄ hollow nanospheres, and methods of making and using thereof.

BACKGROUND

The removal or conversion of nitrate (NO_x⁻ species) in agricultural sewage is highly desired because nitrate causes severe damage to aquatic ecosystems and underground water, and eventually poses a threat to human health. Fortunately, nitrate may be converted into ammonia (NH₃), which plays a significant role in the nitrogen cycle and may be used as a raw material in hydrogen-rich fuels in the industry. There are two typical pathways to produce ammonia: the conventional Haber-Bosch process, which requires complicated chemical reactions between N₂ and H₂ implemented at high temperatures and pressures, and the electrochemical nitrogen reduction reaction (NO₃RR), which, although it may be executed at ambient conditions, has a much lower yield rate than the Haber-Bosch process. The NO₃RR involves eight-electron transfer processes and is competitive with the undesired hydrogen evolution reaction (HER). Therefore, there is a need for electrocatalysts with high selectivity and efficiency toward NH₃/NH₄⁺.

SUMMARY

The present disclosure offers advantages, benefits, and other alternatives over known compositions and methods, by providing electrocatalysts, and methods of making and using thereof, that are highly selective and efficient.

In an aspect, provided is a method of making a catalyst, including: forming a CuWO₄ hollow nanosphere by a hydrothermal process followed by a thermal treatment; plasma-treating the CuWO₄ hollow nanosphere with a plasma to introduce oxygen vacancies; and introducing Mo clusters adjacent to the oxygen vacancies; wherein the hydrothermal process comprises mixing a copper source, a tungsten source, and an adjuvant to form a solution, heating the solution to form a precursor precipitate, and drying the precursor precipitate; and wherein the thermal treatment comprises annealing the precursor precipitate to form a CuWO₄ hollow nanosphere.

In an example, the copper source is a water-soluble copper salt, the tungsten source is a water-soluble tungsten salt, and the adjuvant is sodium citrate dihydrate (C₆H₅O₇Na₃·2H₂O). In another example, a ratio of copper source:adjuvant:tungsten source is in a range of about 1:1:0.5 to about 1:1:1.

In yet another example, heating is performed at a temperature in a range of about 160° C. to about 180° C. for about 22 hours to about 26 hours.

In an example, annealing the precursor precipitate is performed at a temperature in a range of about 350° C. to about 450° C. for about 1 hour to about 2hours.

In another example, plasma-treating includes one or both of (i) exposing the CuWO₄ hollow nanosphere to a plasma for about 300 seconds to about 600 seconds; and (ii) exposing the CuWO₄ hollow nanosphere to a plasma with RF power about 200 W and gas flow in a range of about 10 sccm to about 20 sccm. In yet another example, the plasma is Ar or N₂ and the CuWO₄ hollow nanosphere has a high concentration of oxygen vacancies. In still another example, the plasma is O₂ and the CuWO₄ hollow nanosphere has a low concentration of oxygen vacancies.

In a further example, introducing Mo clusters includes (i) mixing polyoxomolybdate and the plasma-treated CuWO₄ hollow nanosphere in water and evaporating the solution to obtain a powder comprising the plasma-treated CuWO₄ hollow nanosphere with Mo cluster inclusions; and (ii) annealing the powder at a temperature in a range of about 80° C. to about 100° C. for about 1 hour to about 2 hours in H₂ /Ar atmosphere.

In an aspect, provided is a catalyst including: a CuWO₄ hollow nanosphere; asymmetric oxygen vacancies within the CuWO₄ hollow nanosphere; and Mo clusters within the CuWO₄ hollow nanosphere, wherein the Mo clusters are adjacent to the asymmetric oxygen vacancies. In an example, the concentration of asymmetric oxygen vacancies is low. In another example, the concentration of asymmetric oxygen vacancies is high.

In yet another example, the diameter of the CuWO₄ hollow nanosphere is in a range of about 300 nm to about 450 nm.

In still another example, the catalyst is made by a method including: forming a CuWO₄ hollow nanosphere by a hydrothermal process followed by a thermal treatment; plasma-treating the CuWO₄ hollow nanosphere with a plasma to introduce oxygen vacancies; and introducing Mo clusters adjacent to the oxygen vacancies; wherein the hydrothermal process comprises mixing a copper source, a tungsten source, and an adjuvant to form a solution, heating the solution to form a precursor precipitate, and drying the precursor precipitate; and wherein the thermal treatment comprises annealing the precursor precipitate to form a CuWO₄ hollow nanosphere.

In an aspect, provided is a method of preparing an electrode for nitrate reduction, the method including: forming a solution comprising a plurality of CuWO₄ hollow nanospheres and Nafion; sonicating the solution to form a homogeneous catalyst ink; and applying the catalyst ink to a support to obtain the electrode. In an example, the solution further includes one or more of ethanol, acetone, or water, or any combination thereof. In another example, the electrode exhibits one or both of (i) a high NH₃ Faradaic efficiency of about 94.60±3.75%, and (ii) a yield rate of about 5.84±0.45 mg h⁻¹ mg_cat.⁻¹ at −0.7 V versus RHE.

In yet another example, the CuWO₄ hollow nanospheres includes: CuWO₄, wherein each O atom is linked to at least one W atom and at least one Cu atom; asymmetric oxygen vacancies; and Mo clusters, wherein the Mo clusters are adjacent to the asymmetric oxygen vacancies. In still another example, the concentration of asymmetric oxygen vacancies is low. In a further example, the concentration of asymmetric oxygen vacancies is high.

BRIEF DESCRIPTION OF THE FIGURES

These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of a method of making a hollow nanosphere;

FIGS. 2A-2F are scanning electron microscopy (SEM) images with various magnifications of CuWO₄ prepared by different hydrothermal durations; FIG. 2A—6 hours, 2 μm; FIG. 2B—6 hours, 500 nm; FIG. 2C—24 hours, 2 μm; FIG. 2D—24 hours, 500 nm; FIG. 2E—48 hours, 2 μm; FIG. 2F—48 hours, 500 nm;

FIG. 3A and FIG. 3B show the hollow structure of CuWO₄; FIG. 3A is a transmission electron microscopy (TEM) image of Cu; and FIG. 3B is a high-resolution transmission electron microscopy (HRTEM) image of CuW;

FIG. 4A is a Fourier-transform infrared spectroscopy (FT-IR) spectra of Cu₂WO₄(OH)₂ and CuWO₄ prepared by different hydrothermal durations;

FIG. 4B depicts a typical symmetric and anti-symmetric stretching vibration of (b) O—Cu—O bonds in octahedral [CuO₆] clusters;

FIG. 4C depicts a typical symmetric and anti-symmetric stretching vibration of (c) O—W—O bonds in octahedral [WO₆] clusters;

FIGS. 5A-5D are SEM images of L—CuW (FIG. 5A and FIG. 5C) and H—CuW (FIG. 5B and FIG. 5D), where CuW are prepared by different hydrothermal durations of 6 hours (FIG. 5A and FIG. 5B) and 24 hours (FIG. 5C and FIG. 5D);

FIG. 6A is an electron paramagnetic resonance (EPR) spectrum of L—CuW, CuW, and H—CuW;

FIG. 6B is an O 1s x-ray photoelectron spectroscopy (XPS) spectrum of L—CuW, CuW, and H-CuW;

FIG. 6C is a Raman spectrum of L—CuW, CuW, and H—CuW;

FIG. 7 depicts x-ray powder diffraction (XRD) patterns of the various CuW nanospheres;

FIG. 8A is an SEM image of Mo/H—CuW, with an inset of an SEM image of single hollow nanosphere and the particle size distribution;

FIG. 8B is a TEM image of Mo/H—CuW;

FIG. 8C is an HRTEM image of Mo/H—CuW, with an inset of a selected area diffraction (SAED) pattern of Mo/H—CuW and boxes showing amplified HRTEM images of the corresponding Inverse Fast Fourier Transition (IFFT) images from the circled regions in the inset;

FIG. 9A is a Cu 2p spectra for CuW, Mo/L—CuW, and Mo/H—CuW;

FIG. 9B is a W 4f spectra for CuW, Mo/L—CuW, and Mo/H—CuW;

FIG. 9C is a Mo 3d XPS spectra for the Mo/L—CuW and Mo/H—CuW;

FIG. 10A depicts linear sweep voltammetry (LSV) curves of Mo/H—CuW, Mo/L—CuW, H—CuW, L—CuW, and CuW in 0.5 M Na₂SO₄ electrolyte with and without NO₃⁻ upon a scan rate of 5 mV s⁻¹;

FIG. 10B depicts NH₃ yield rate and Faradaic efficiency of L—CuW, CuW, and H—CuW;

FIG. 10C depicts NH₃ yield rate and Faradaic efficiency of H—CuW and Mo/H—CuW; and

FIG. 11 depicts the results of a stability test on Mo/H—CuW and H—CuW.

It should be appreciated that all combinations of the foregoing concepts and additional concepts discussed in greater detail below (provided such concepts are not mutually inconsistent) are contemplated as being part of the inventive subject matter disclosed herein and may be used to achieve the benefits and advantages described herein.

DETAILED DESCRIPTION

According to an aspect of the present disclosure, there is provided a method of making a CuWO₄ hollow nanosphere, including forming a CuWO₄ hollow nanosphere by a hydrothermal process followed by a thermal treatment; plasma-treating the CuWO₄ hollow nanosphere with a plasma to introduce oxygen vacancies, and introducing Mo clusters adjacent to the oxygen vacancies. In an example, the hydrothermal process is a facile hydrothermal process that includes mixing a copper source, a tungsten source, and an adjuvant to form a solution; heating the solution to form a precursor precipitate; and drying the precursor precipitate. In another example, the thermal treatment includes annealing the precursor precipitate to form a CuWO₄ hollow nanosphere.

Synthesis of a CuWO₄ Hollow Nanosphere

The CuWO₄ hollow nanosphere may be synthesized by a facile hydrothermal method followed by a thermal treatment, as shown in FIG. 1. In an example, the facile hydrothermal method may include forming a solution including a copper source, a tungsten source, and an adjuvant; heating the solution to form a precursor precipitate; and collecting and washing the precursor precipitate.

In another example, forming a solution may include mixing the copper source, the tungsten source, and the adjuvant in an aqueous liquid to form a homogeneous solution. In yet another example, forming a solution may include dissolving the copper source and the tungsten source in an aqueous liquid to form a homogeneous solution, followed by the addition of the adjuvant. In still another example, the aqueous liquid may be deionized water. In a further example, the copper source may be a water-soluble copper salt. Non-limiting examples of water-soluble copper salts include copper chloride dihydrate (CuCl₂·2H₂O) and copper nitrate hydrate (Cu(NO₃)₂·3H₂O). In yet a further example, the tungsten source may be a water-soluble tungsten salt. Non-limiting examples of water-soluble tungsten salts include sodium tungstate dihydrate (Na₂WO₄·2H₂O) and ammonium tungstate hydrate ((NH₄)₆W₇O₂₄·6H₂O). In still a further example, the adjuvant may be sodium citrate dihydrate (C₆H₅O₇Na₃·2H₂O). The adjuvant may be used to control the growth of the hollow nanosphere structures. In another further example, the ratio of copper source:adjuvant:tungsten source may be in a range of about 1:1:0.5 to about 1:1:1, including all ranges and subranges therein, e.g., about 1:1:0.5 to about 1:1:0.6, about 1:1:0.6 to about 1:1:0.7, about 1:1:0.7 to about 1:1:0.8, about 1:1:0.8 to about 1:1:0.9, about 1:1:0.9 to about 1:1:1, etc.

In another example, heating the solution to form a precursor precipitate may include heating the solution to a first temperature in a range of about 160° C. to about 180° C., including all ranges and subranges therein, e.g., about 160° C. to about 170° C., about 170° C. to about 180° C., about 160° C. to about 165° C., about 165° C. to about 170° C., about 170° C. to about 175° C., about 175° C. to about 180° C., etc. In yet another example, heating the solution to form a precursor precipitate may include heating the solution for about 22 hours to about 26 hours, including all ranges and subranges therein, e.g., about 22 hours to about 24 hours, about 24 hours to about 26 hours, about 22 hours to about 23 hours, about 23 hours to about 24 hours, about 24 hours to about 25 hours, about 25 hours to about 26 hours, about 22 hours to about 22.5 hours, about 22.5 hours to about 23 hours, about 23 hours to about 23.5 hours, about 23.5 hours to about 24 hours, about 24 hours to about 24.5 hours, about 24.5 hours to about 25 hours, about 25 hours to about 25.5 hours, about 25.5 hours to about 26 hours, etc.

In another example, collecting and washing the precursor precipitate may include collecting the precursor precipitate by centrifugation. In yet another example, collecting and washing the precursor precipitate may include washing the precursor precipitate with deionized water and ethanol.

In another example, the thermal treatment may include annealing the precursor precipitate to form a CuWO₄ hollow nanosphere. In yet another example, the precipitate precursor is annealed at a temperature in a range of about 350° C. to about 400° C., including all ranges and subranges therein, e.g., about 350° C. to about 360° C., about 360° C. to about 370° C., about 370° C. to about 380° C., about 380° C. to about 390° C., about 390° C. to about 400° C., about 350° C. to about 355° C., about 355° C. to about 360° C., about 360° C. to about 365° C., about 365° C. to about 370° C., about 370° C. to about 375° C., about 375° C. to about 380° C., about 380° C. to about 385° C., about 385° C. to about 390° C., about 390° C. to about 395° C., about 395° C. to about 400° C., etc.

For example, the facile hydrothermal method may include dissolving a copper source and a tungsten source in deionized water to form a homogeneous solution, followed by the addition of an adjuvant. The solution may then be heated at a temperature in a range of about 160° C. to about 180° C. for about 22 hours to about 26 hours, resulting in a precipitate of the Cu₂WO₄(OH)₂ precursor. The precursor precipitate may be collected by centrifugation and washed with deionized water and ethanol. Then the precursor precipitate may be subjected to a thermal treatment to yield a CuWO₄ hollow nanosphere. For example, the precursor precipitate may be annealed at a temperature in a range of about 350° C. to about 450° C. for about 1 hour to about 2 hours in air atmosphere to form a CuWO₄ hollow nanosphere.

The morphology of CuWO₄ may be controlled by varying the duration of the hydrothermal process. In an example, the hydrothermal process may occur for about 4 hours to about 8 hours, including all ranges and subranges therein, e.g., about 4 hours to about 6 hours, about 6 hours to about 8 hours, about 4 hours to about 5 hours, about 5 hours to about 6 hours, about 6 hours to about 7 hours, about 7 hours to about 8 hours, about 4 hours to about 4.5 hours, about 4.5 hours to about 5 hours, about 5 hours to about 5.5 hours, about 5.5 hours to about 6 hours, about 6 hours to about 6.5 hours, about 6.5 hours to about 7 hours, about 7.5 hours to about 8 hours, etc., to form a solid CuWO₄ nanosphere with an average diameter in a range of about 216 nm to about 394 nm, including all ranges and subranges therein, e.g., about 216 nm to about 300 nm, about 300 nm to about 394 nm, about 216 nm to about 250 nm, about 250 nm to about 300 nm, about 300 nm to about 350 nm, about 350 nm to about 394 nm, etc., (examples shown in FIG. 2A and FIG. 2B). In another example, the hydrothermal process may occur for about 22 hours to about 26 hours, including all ranges and subranges therein, e.g., about 22 hours to about 26 hours, including all ranges and subranges therein, e.g., about 22 hours to about 24 hours, about 24 hours to about 26 hours, about 22 hours to about 23 hours, about 23 hours to about 24 hours, about 24 hours to about 25 hours, about 25 hours to about 26 hours, about 22 hours to about 22.5 hours, about 22.5 hours to about 23 hours, about 23 hours to about 23.5 hours, about 23.5 hours to about 24 hours, about 24 hours to about 24.5 hours, about 24.5 hours to about 25 hours, about 25 hours to about 25.5 hours, about 25.5 hours to about 26 hours, etc. to form hollow CuWO₄ nanospheres with an average diameter in a range of about 300 nm to about 450 nm, including all ranges and subranges therein, e.g., about 300 nm to about 350 nm, about 350 nm to about 400 nm, about 400 nm to about 450 nm, about 300 nm to about 325 nm, about 325 nm to about 350 nm, about 350 nm to about 375 nm, about 375 nm to about 400 nm, about 400 nm to about 425 nm, about 425 nm to about 450 nm, etc. (examples shown in FIG. 2C and FIG. 2D). Moreover, if the hydrothermal process occurs for about 46 hours to about 50 hours, including all ranges and subranges therein, e.g., about 46 hours to about 48 hours, about 48 hours to about 50 hours, about 46 hours to about 47 hours, about 47 hours to about 48 hours, about 48 hours to about 49 hours, about 49 hours to about 50 hours, etc., the CuWO₄ hollow nanospheres may begin to break, resulting in an average diameter in a range of about 303 nm to about 441 nm, including all ranges and subranges therein, e.g., about 303 nm to about 350 nm, about 350 nm to about 400 nm, about 400 nm to about 441 nm, etc., (examples shown in FIG. 2E and FIG. 2F).

In an example, the hydrothermal process occurs for about 6 hours and results in CuWO₄ solid nanospheres with an average diameter of about 305.7 nm. In another example, the hydrothermal process occurs for about 24 hours and results in CuWO₄ hollow nanospheres with an average diameter of about 373.6 nm. In yet another example, the hydrothermal process occurs for about 48 hours and results in CuWO₄ broken nanospheres with an average diameter of about 372.2 nm.

FIG. 3A, which depicts a transmission electron microscopy image of CuW, and FIG. 3B, which depicts a high-resolution transmission electron microscopy image of CuW, show an example of the hollow structure of CuWO4, and the lattice fringes with the interplanar spacings of 0.310 nm correspond to the (111) plane of triclinic CuWO₄.

FIG. 4A, which depicts a Fourier-transform infrared spectroscopy spectra, shows an example of the transformation from Cu₂WO₄(OH)₂ precursor to CuWO₄ during the thermal treatment. The band at approximately 470 cm⁻¹ represents the symmetric stretching vibrations of [CuO₆] and the other peaks (at approximately 908, 710, 554, and 420 cm⁻¹) represent the symmetric stretching, anti-symmetric stretching, interactive symmetric stretching and symmetric bending vibrations of the distorted [WO₆] clusters. This information demonstrates the distortions on octahedral [CuO₆] and [WO₆] clusters and suggests the degree of asymmetry Cu—O_v—W sites.

Synthesis of CuWO₄ Hollow Nanospheres with Different Concentrations of Ov

Oxygen vacancies (O_v) are generally considered active sites, which are important factors dictating the activity of electrocatalysts, and give rise to an electron-rich surface that lowers the adsorption/activation energy of the target molecule. Symmetric O_v represents the site chained with the symmetric coordinated cations, whereas the linkage terminals of ones constituted by different kinds of cations are called asymmetric O_v. Asymmetric O_v keeps a dynamic balance between the adsorption and the desorption of oxygen species.

Plasma atmosphere treatment may be used to introduce various concentrations of O_v into CuWO₄. In an example, the CuWO₄ hollow nanosphere may be exposed to a plasma with RF power of about 200 W and gas flow in a range of about 10 sccm to about 20 sccm, including all ranges and subranges therein, e.g., about 10 sccm to about 15 sccm, about 15 sccm to about 20 sccm, about 10 sccm to about 12 sccm, about 12 sccm to about 14 sccm, about 14 sccm to about 16 sccm, about 16 sccm to about 18 sccm, about 18 sccm to about 20 sccm, etc. In another example, CuWO₄ hollow nanospheres may be exposed to a plasma for about 300 seconds to about 600 seconds, including all ranges and subranges therein, e.g., about 300 seconds to about 300 seconds, about 400 seconds to about 500 seconds, about 500 seconds to about 600 seconds, about 300 seconds to about 350 seconds, about 350 seconds to about 400 seconds, about 400 seconds to about 450 seconds, about 450 seconds to about 500 seconds, about 500 seconds to about 550 seconds, about 550 seconds to about 600 seconds, about 300 seconds to about 320 seconds, about 320 seconds to about 340 seconds, about 340 seconds to about 360 seconds, about 360 seconds to about 380 seconds, about 380 seconds to about 400s seconds, about 400 seconds to about 420 seconds, about 420 seconds to about 440 seconds, about 440 seconds to about 460 seconds, about 460 seconds to about 480 seconds, about 480 seconds to about 500 seconds, about 500s seconds to about 520 seconds, about 520 seconds to about 540 seconds, about 540 seconds to about 560 seconds, about 560 seconds to about 580 seconds, about 580 seconds to about 600 seconds, etc. In yet another example, the plasma atmosphere may be Ar or N₂. When Ar or N₂ plasma bombards the surface of CuWO₄, the internal energy of metastable Ar-plasma may be transferred to the surface atoms, which leads to the removal of the relatively light oxygen atoms and results in CuWO₄ with a high concentration of O_v (H—CuW). In a further example, the plasma atmosphere may be O₂. The O₂ plasma-induced oxygen ions or radical groups may be trapped inside CuWO₄ to fill O_v, resulting in CuWO₄ with a low concentration of O_v (L—CuW).

In an example, different concentrations of oxygen vacancies may be introduced by annealing the CuWO₄ hollow nanosphere in hydrogen atmosphere at a temperature of about 300° C. for about 20 minutes to about 60 minutes, including all ranges and subranges therein, e.g., about 20 minutes to about 40 minutes, about 40 minutes to about 60 minutes, about 20 minutes to about 30 minutes, about 30 minutes to about 40 minutes, about 40 minutes to about 50 minutes, about 50 minutes to about 60 minutes, etc. In an example, the hydrogen atmosphere may be H₂/Ar 5% vol.

Every O atom may be linked to at least one W atom and one Cu atom in CuWO₄, thus the oxygen vacancies in CuWO₄ may build a Cu—O_v—W asymmetric structure, in which the two surrounding cations with different electronegativity own opposite influences toward the stability of oxygen species. Asymmetric O_v balances the adsorption and desorption of oxygen species, thus enhancing the performance of NO₃RR.

Plasma atmosphere treatment did not affect the morphology of the CuWO₄ nanospheres, as indicated by examples shown in FIGS. 5A through 5D. For example, CuWO₄ solid nanospheres (e.g., hydrothermal method duration of about 6 hours) treated with O₂ plasma (FIG. 5A) or Ar plasma (FIG. 5B) retain their solid nanosphere morphology. In another example, CuWO₄ hollow nanospheres (e.g., hydrothermal method duration of about 24 hours) treated with O₂ plasma (FIG. 5C) or Ar plasma (FIG. 5D) retain their solid or hollow nanosphere morphology.

O_v concentrations were assessed by electron paramagnetic resonance (EPR) spectroscopy, x-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Signals in a range of about g=2.001 to about g=2.003, including all ranges and subranges therein, e.g., about 2.001 to about 2.002, about 2.002 to about 2.003, etc., on an EPR spectrum may suggest the presence of oxygen vacancies. In an example, a signal at about g=2.002 on the EPR spectra of L—CuW, CuW, and H—CuW, suggests the presence of oxygen vacancies (FIG. 6A).

Furthermore, on a high-resolution XPS spectra of O 1s, a peak in a range of about 530.1 eV to about 530.3 eV, including all ranges and subranges therein, e.g., about 530.1 eV to about 530.2 eV, about 530.2 eV to about 530.3 eV, etc., may indicate lattice oxygen and a peak in a range of about 531.1 eV to about 531.3 eV, including all ranges and subranges therein, e.g., about 531.1 eV to about 531.2 eV, about 531.2 eV to about 531.3 eV, etc., may indicate oxygen atoms next to O_v. In an example, a peak at about 530.2 eV on the high-resolution XPS spectra of L—CuW, CuW, and H—CuW, may indicate lattice oxygen (B in FIG. 6B). In another example, a peak at about 531.2 eV on the high-resolution XPS spectra of L—CuW, CuW, and H—CuW, may indicate oxygem atoms next to Ov (A in FIG. 6B).

A higher A/B ratio suggests a higher concentration of O_v upon the bombardment of plasma on the surface of the CuWO₄ nanospheres. For example, H—CuW has a higher A/B ratio compared to CuW and L—CuW, which may suggest a higher concentration of O_v upon Ar plasma treatment compared to no treatment and O₂ treatment.

FIG. 6C shows example Raman spectra for L—CuW, CuW, and H—CuW, and the Raman bands are assigned to the Raman-active vibrational modes (Ag) of triclinic CuWO₄. The Raman bands of H—CuW are weaker and broader compared to CuW and L—CuW, which may indicate a decrease of (O—Cu—O)/(O—W—O) bonds and the generation of more O_v in CuW. Therefore, plasma atmosphere treatment using Ar plasma results in a higher concentration of O_v compared to using O₂, which removes the O_v.

Synthesis of a CuWO₄ Hollow Nanosphere with Mo Clusters

Transition metal clusters such as polyoxometalates (POMs), with bridged metal-metal interactions, can minimize the distance between single-atom centers, thereby improving the intrinsic activity and stability compared to single-atom catalyst (SACs) and maximizing the efficiency of metal atoms. Metal clusters may have unoccupied d-orbitals, which can accept multiple electrons, resulting in the modulation of charge transfer when binding to NO₃⁻.

A Mo source may be introduced into CuWO₄ to orientate the Mo cluster adjacent to the O_v. The asymmetric O_v may facilitate a dynamic balance between the adsorption and desorption of O in NO₃⁻. Further, the Mo clusters may promote the protonation process. A synergistic effect between the asymmetric O_v and adjacent Mo clusters may result in CuWO₄ hollow nanospheres that exhibit improved NH₃ Faradaic efficiency of 94.60±3.75% and yield rate of about 5.84±0.45 mg h⁻¹ mg_cat.⁻¹ at −0.7 V versus RHE.

Introducing Mo clusters into a CuWO₄ hollow nanosphere may include dissolving the plasma-treated CuWO₄ hollow nanosphere with a Mo source in water to form a solution, evaporating at a temperature in a range of about 80° C. to about 100° C., including all ranges and subranges therein, e.g., about 80° C. to about 90° C., about 90° C. to about 100° C., about 80° C. to about 85° C., about 85° C. to about 90° C., about 90° C. to about 95° C., about 95° C. to about 100° C., etc., for about 8 hours to about 16 hours, including all ranges and subranges therein, e.g., about 8 hours to about 12 hours, about 12 hours to about 16 hours, about 8 hours to about 10 hours, about 10 hours to about 12 hours, about 12 hours to about 16 hours, about 8 hours to about 9 hours, about 9 hours to about 10 hours, about 10 hours to about 11 hours, about 11 hours to about 12 hours, about 12 hours to about 13 hours, about 13 hours to about 14 hours, about 14 hours to about 15 hours, about 15 hours to about 16 hours, etc. to obtain a powder including a plasma-treated CuWO₄ hollow nanosphere with Mo cluster inclusions. Introducing Mo clusters may further include annealing the powder at a temperature in a range of about 80° C. to about 100° C., including all ranges and subranges therein, e.g., about 80° C. to about 90° C., about 90° C. to about 100° C., about 80° C. to about 85° C., about 85° C. to about 90° C., about 90° C. to about 95° C., about 95° C. to about 100° C., etc., for about 1 hour to about 2 hours, including all range and subranges therein, e.g., about 1 hour to about 1.5 hours, about 1.5 hours to about 2 hours, about 1 hour to about 1.2 hours, about 1.2 hours to about 1.4 hours, about 1.4 hours to about 1.6 hours, about 1.6 hours to about 1.8 hours, about 1.8 hours to about 2 hours, about 1 hour to about 1.1 hours, about 1.1 hours to about 1.2 hours, about 1.2 hours to about 1.3 hours, about 1.3 hours to about 1.4 hours, about 1.4 hours to about 1.5 hours, about 1.5 hours to about 1.6 hours, about 1.6 hours to about 1.7 hours, about 1.7 hours to about 1.8 hours, about 1.8 hours to about 1.9 hours, about 1.9 hours to about 2 hours, etc., in H₂/Ar atmosphere. In an example, the Mo source is a polyoxomolybdate. Non-limiting examples of polyoxomolybdates include ammonium paramolybdate ((NH₄)₆Mo₇O₂₄·4H₂O), ammonium octamolybdate ((NH₄)₄Mo₈O₂₆)), and the like. In another example, the annealing occurs at about 100° C. for about 1 hour.

In an example, ammonium paramolybdate was introduced into H—CuW and L—CuW to orientate the Mo cluster next to the O_v. Then, the oxygen atoms trapped in the vacancy sites were removed through reduction under H₂/Ar atmosphere at a low temperature. X-ray powder diffraction (XRD) was performed on all samples, e.g., CuW, L—CuW, H—CuW, Mo/L—CuW, and Mo/H—CuW, (examples shown in FIG. 7) and indicates that each is indexed to triclinic phase CuWO₄ (JCPDS no. 72-0616) without any other detectable impurities.

In another example, the CuWO₄ hollow nanosphere exhibits a high NH₃ Faradaic efficiency of about 94.60±3.75% and yield rate of about 5.84±0.45 mg h⁻¹ mg_cat.⁻¹ at −0.7 V versus RHE.

An example of the structure of a Mo/H—CuW hollow nanosphere is shown in FIGS. 8A through 8C. In FIG. 8A, the SEM image depicts examples of Mo/H—CuW as hollow nanospheres, about 377 nm in diameter, with the shell composed of numerous particles. Insets of FIG. 8A depict a single hollow nanosphere and the particle size distribution.

In FIG. 8B, the dark and light boundaries in the TEM image of an example Mo/H—CuW further confirm the hollow structure of the Mo/H—CuW. Generally, hollow-structured nanomaterials such as these have larger specific surface areas than solid ones, thereby enabling the exposure of more active sites and promoting electrocatalytic activity. Further, there are more interfaces among the thin shells because of the numerous small particles of the CuW hollow nanospheres, which provides fast channels for mass transport.

In FIG. 8C, the HRTEM image of an example Mo/H—CuW depicts the lattice fringed with interplanar spacings of about 0.310 nm, about 0.382 nm, and about 0.252 nm, which correspond to the (111) (110) and (021) planes of triclinic CuW, respectively. Most of the lattice fringes are inconsecutive in the images of Mo/H—CuW. After applying the inverse fast Fourier transform (IFFT) to the selected region, dislocations and distortions were observed, indicating the introduction of abundant oxygen vacancies into the lattice due to the plasma treatment and Mo doping. These oxygen vacancies enable an increase in active sites. In addition, the SAED pattern (inset of FIG. 8C) suggests the characteristics of polycrystalline triclinic CuW.

In FIG. 9A, the peaks of Cu 2p_3/2and Cu 2p_1/2 of CuW and Mo/L—CuW locate at similar positions whereas the peaks of Cu 2p_3/2and Cu 2p_1/2 of Mo/H—CuW are shifted to a lower binding energy. This indicates an increase in local electron density of Cu due to delocalized electrons in the O_v.

In FIG. 9B, the peaks of W 4f of Mo/H—CuW shift to higher binding energies compared to CuW and Mo/L—CuW. This demonstrates the electron transfer from W to Mo clusters and the formation of W—O—Mo bonds with the help of Cu—O_v—W asymmetric sites. Therefore, the Cu—O_v—W asymmetric sites may modulate the local charge distribution and then contribute to polarizing the adsorbed NO₃⁻ molecules for better activation and surface electrochemical nitrate reduction process.

In FIG. 9C, for Mo/H—CuW, the deconvoluted peaks at 232.04 eV, 235.16 eV and 233.07 eV, 236.33 eV can be indexed to Mo⁵⁺ and Mo⁶⁺, respectively. However, only Mo⁶⁺ exists in the sample of Mo/L—CuW. As for the VI valence state of Mo in the pristine POMs, the V valence state may result due to the charge transfer along with the W—O—Mo bonds.

Thus, plasma treatment may regulate Cu—O_v—W asymmetric sites and Mo clusters are accurately located in CuWO₄. The electrochemical activity of the electrocatalyst may be enhanced and stabilized by combining the electron-donating O_v and the electron-compensating Mo cluster.

The characteristics of a two-compartment H-type electrolytic cell were evaluated under ambient conditions to examine the synergistic effect of O_v and Mo clusters towards NO₃RR performance. FIG. 10A depicts a linear sweep voltammetry (LSV) curve for each of CuW, L—CuW, H—CuW, Mo/L—CuW, and Mo/H—CuW in the Na₂SO₄ electrolyte with and without NO₃⁻. The current densities of all the samples for NO₃RR are distinctly higher over a wide range of negative potentials than the ones for HER, indicating the electrocatalytic activity for NO₃⁻ reduction and the poor activity on HER, which enables the high selectivity for NH₃ production.

To obtain the NH₃ yield rate and FE, a series of controlled-potential CA measurements were carried out in NO₃—contained electrolyte. As shown in FIG. 10B, H—CuW hollow nanospheres achieved the highest NH₃ FE of 75.61% with a yield rate of 3.63 mg h⁻¹ mg_cat.⁻ at −0.70 V vs. RHE, significantly outperforming L—CuW (FE of 41.63% and yield rate of 1.63 mg h⁻¹ mg_cat.⁻) and CuW HNS (FE of 49.25% and yield rate of 2.14 mg h⁻¹ mg_cat.⁻¹). The highest FE of H—CuW appears at a more positive potential than the counterparts, which means it only needs a smaller voltage to achieve the higher NO₃⁻ reduction efficiency, emphasizing the selectivity of CuW with high concentration of O_v sites

As shown in FIG. 10C, in connection with the effect of potential change, the NH₃ yield rate of Mo/H—CuW gradually increases as the reduction potential increases from −0.4 V to −1.0 V, offering the NH₃yield rate of 5.84±0.45 mg h⁻¹ mg_cat.⁻¹ at −0.7 V vs. RHE. The highest FE reaches 94.60±3.75% at −0.7 V vs. RHE, surpassing those of H—CuW and L—CuW by a factor of 1.25 and 2.27, respectively.

Referring now to FIG. 11, the NH₃FE for H—CuW shows an apparent descending tendency after 15 cycles compared to Mo/H—CuW. It is inferred that the reduction is caused by the fugitiveness of O_v. Although the asymmetric O_v could theoretically balance the adsorption/desorption of O in NO₃⁻, the practical experiments of H—CuW indicate that it has a relatively slow kinetic for the eight-electron transfer reduction process. Thus, less and less O_v is left to absorb additional NO₃⁻ due to the fugitiveness as the number of cycles increases. However, when loading the Mo clusters onto H—CuW, the NH₃FE fluctuates but still maintains at around 90%, confirming the synergistic effect between asymmetric O_v and Mo clusters. It is known that the bridge or hollow site adsorption is available on the Mo metal surface for the strong *N binding strength and relatively stable *H. Therefore, this synergistic effect could not only promote the NO₃RR/HER selectivity but also accelerate the protonation steps in NO₃RR. This is why the O_v-only H—CuW displays the decreasing efficiency after repetitive use, but the Mo/H—CuW does not.

According to another aspect of the present disclosure, there is provided a method of making a catalyst, including forming a CuWO₄ hollow nanosphere by a hydrothermal process followed by a thermal treatment, plasma-treating the CuWO₄ hollow nanosphere with a plasma to introduce oxygen vacancies, and introducing Mo clusters; wherein the hydrothermal process includes mixing a copper source, a tungsten source, and an adjuvant to form a solution; heating the solution to form a precursor precipitate; and drying the precursor precipitate. In an example, the thermal treatment includes annealing the precursor precipitate to form a CuWO₄ hollow nanosphere.

According to another aspect of the present disclosure, there is provided a method of making a catalyst ink, including forming a solution including a plurality of CuWO₄ hollow nanospheres, Nafion; and sonicating the solution to form a homogeneous catalyst ink. In an example, the solution further includes one or more of ethanol, acetone, or water, or any combination thereof. In an example, a first solution may be prepared including deionized water:ethanol in about a 1:3 ratio by volume. In another example, a second solution may be prepared including 40 μL of 5 wt. % Nafion and 960 μL of the first solution. In yet another example, a second solution may be prepared including 40 μL of 5 wt. % Nafion and 960 μL of ethanol. In still another example, a second solution may be prepared including 40 μL of 5 wt. % Nafion and 960 μL of acetone. In an example, catalyst is added to the second solution and then sonicated in an ultrasonic cleaning machine to form a homogeneous catalyst ink. In another example, the amount of catalyst added to the second solution is in a range of about 5 mg to about 10 mg, including all ranges and subranges therein, e.g., about 5 mg to about 7 mg, about 6 mg to about 8 mg, about 7 mg to about 9 mg, about 8 mg to about 10 mg, about 5 mg to about 6 mg, about 6 mg to about 7 mg, about 7 mg to about 8 mg, about 8 mg to about 9 mg, about 9 mg to about 10 mg, etc. In yet another example, the sonicating occurs for about 30 minutes to about 90 minutes, including all ranges and subranges therein, e.g., about 30 30 minutes to about 60 minutes, about 60 minutes to about 90 minutes, about 30 minutes to about 45 minutes, about 45 minutes to about 60 minutes, about 60 minutes to about 75 minutes, about 75 minutes to about 90 minutes, about 30 minutes to about 40 minutes, about 40 minutes to about 50 minutes, about 50 minutes to about 60 minutes, about 60 minutes to about 70 minutes, about 70 minutes to about 80 minutes, about 80 minutes to about 90 minutes, etc.

A CuWO₄ Hollow Nanosphere

According to an aspect of the present disclosure, there is provided a composition including a CuWO₄ hollow nanosphere; asymmetric oxygen vacancies within the CuWO₄ hollow nanosphere; and Mo clusters within the CuWO₄ hollow nanosphere, wherein the Mo clusters are adjacent to the asymmetric oxygen vacancies.

The concentration of asymmetric oxygen vacancies in the CuWO₄ hollow nanosphere may be controlled during synthesis by treating the CuWO₄ hollow nanosphere with plasma for a specific length of time. Non-limiting examples of plasma include O₂ and Ar. In an example, the CuWO₄ hollow nanosphere is exposed to Ar plasma, thereby increasing the concentration of oxygen vacancies. In another example, the CuWO₄ hollow nanosphere is exposed to O₂ plasma, thereby decreasing the concentration of oxygen vacancies. In yet another example, the length of time that the CuWO₄ hollow nanosphere is exposed to plasma may be in a range of about 300 seconds to about 600 seconds, including all ranges and subranges therein, e.g., about 300 seconds to about 330 seconds, about 330 seconds to about 360 seconds, about 360 seconds to about 390 seconds, about 390 seconds to about 420 seconds, about 420 seconds to about 450 seconds, about 450 seconds to about 480 seconds, about 480 seconds to about 510 seconds, about 510 seconds to about 540 seconds, about 540 seconds to about 570 seconds, about 570 seconds to about 600 seconds, etc.

In still another example, the diameter of the CuWO₄ hollow nanosphere may be in a range of about 300 nm to about 450 nm, including all ranges and subranges therein, e.g., about 300 nm to about 235 nm, about 325 nm to about 350 nm, about 350 nm to about 375 nm, about 375 nm to about 400 nm, about 400 nm to about 425 nm, about 425 nm to about 450 nm, etc.

In still another example, the CuWO₄ hollow nanosphere may index to triclinic phase CuWO₄ without any other detectable impurities.

In an example, the CuWO₄ hollow nanosphere may be made by any one of the methods described hereinabove.

In another example, the CuWO₄ hollow nanosphere may be used as a catalyst.

According to an aspect of the present disclosure, there is provided a catalyst including a plurality of CuWO₄hollow nanospheres; wherein the CuWO₄ hollow nanospheres include asymmetric oxygen vacancies within the CuWO₄ hollow nanosphere; and Mo clusters within the CuWO₄ hollow nanosphere, wherein the Mo clusters are adjacent to the asymmetric oxygen vacancies. The catalyst may be made by any one of the methods described hereinabove. In an example, the catalyst may be used for efficient ammonia electrosynthesis.

In yet another example, the CuWO₄ nanosphere may be used in a catalyst ink.

According to an aspect of the present disclosure, there is provided a catalyst ink including a plurality of CuWO₄ hollow nanospheres, Nafion, ethanol, and water. The plurality of CuWO₄ hollow nanospheres may be made by any one of the methods described hereinabove.

An Electrode Including a Plurality of CuWO₄ Hollow Nanospheres

According to an aspect of the present disclosure, there is provided an electrode including a support and a catalyst ink. In an example, the support may be a clean carbon cloth. In another example, the support may be a glassy carbon electrode. In another example, the carbon cloth may be prepared by washing the cloth with acetone, ethanol, and deionized water. In yet another example, the carbon cloth may be washed multiple times with acetone, ethanol, and deionized water. In yet another example, the carbon cloth may be sonicated in one or more of acetone, ethanol, or deionized water, or any combination thereof. In still another example, the glassy carbon electrode may be polished by abrasive paper for metallograph. In a further example, the glassy carbon electrode may be polished on the suet with the suet including a Al₂O₃ slurry coating. In yet a further example, the glassy carbon electrode may be sonicated in ethanol, diluted HNO₃, and deionized water.

In still another example, the catalyst ink includes a catalyst, Nafion, ethanol, and water, as described above. In a further example, the catalyst includes a plurality of CuWO₄ hollow nanospheres, as described above. In an example, the electrode described in the present disclosure may be used for nitrate reduction.

According to an aspect of the present disclosure, there is provided a method of preparing an electrode for nitrate reduction, including forming a solution comprising a plurality of CuWO₄ hollow nanospheres, Nafion; sonicating the solution to form a homogeneous catalyst ink; and applying the catalyst ink to a support to obtain the electrode. In an example, the solution further includes one or more of ethanol, acetone, or water, or any combination thereof.

In another example, a solution may be prepared including 5 wt. % Nafion and deionized water/ethanol. In yet another example, the ratio of deionized water to ethanol in the solution may be about 1:3 by volume. In still another example, the catalyst may be dispersed in the solution. In a further example, the solution with dispersed catalyst may be sonicated to form a homogeneous catalyst ink. In yet another example, the solution with catalyst is sonicated for about 30 minutes to about 90 minutes, including all ranges and subranges therein, e.g., about 30 minutes to about 60 minutes, about 60 minutes to about 90 minutes, about 30 minutes to about 50 minutes, about 50 minutes to about 70 minutes, about 70 minutes to about 90 minutes, about 30 minutes to about 45 minutes, about 45 minutes to about 60 minutes, about 60 minutes to about 75 minutes, about 75 minutes to about 90 minutes, about 30 minutes to etc.

For example, the solution with dispersed catalyst may be sonicated in an ultrasonic cleaning machine for about 1 h to form a homogeneous catalyst ink. In yet a further example, the catalyst ink is applied to the clean carbon cloth and dried at room temperature. In an example, the carbon cloth may be cleaned with acetone, ethanol, and deionized water one or more times.

In yet a further example, the catalyst ink is applied to the support material. In still a further example, the catalyst ink is dropped onto the support material.

In an example, the electrode exhibits a high NH₃ Faradaic efficiency of about 94.60±3.75%. In another example, the electrode exhibits a yield rate of about 5.84±0.45 mg h⁻¹ mg_cat.⁻¹ at −0.7 V versus RHE.

In yet another example, the CuWO₄ hollow nanospheres include: CuWO₄, wherein each O atom is linked to at least one W atom and at least one Cu atom; asymmetric oxygen vacancies; and Mo clusters, wherein the Mo clusters are adjacent to the asymmetric oxygen vacancies. In still another example, the concentration of asymmetric oxygen vacancies is low. In a further example, the concentration of asymmetric oxygen vacancies is high.

EXAMPLES

Synthesis of CuWO₄ Hollow Nanospheres

In an example, copper chloride dihydrate (CuCl₂·2H₂O) and sodium tungstate dihydrate (Na₂WO₄·2H₂O) acted as the source of Cu and W, and sodium citrate dihydrate (C₆H₅O₇Na₃·2H₂O) was used as an adjuvant to control the growth of hollow nanospheres structure. In an example, 50 mM CuCl₂·2H₂O and 50 mM C₆H₅O₇Na₃·2H₂O were dissolved in deionized water under magnetic stirring to form a homogeneous solution, followed by adding 25 mM Na₂WO₄·2H₂O. The homogeneous solution was poured into 100 mL sealed Teflon-lined autoclave and then transferred to an oven set at 160° C. After a certain time (6 h, 24 h, and 48 h) for hydrothermal reaction, the precipitate in the solution was collected by centrifugation and washed with deionized water and ethanol for several times. The obtained green powder (Cu₂WO₄(OH)₂) was finally annealed at 400° C. for 1 h in air atmosphere to be oxidized into brown powders (CuWO₄).

Synthesis of CuWO₄ Hollow Nanospheres with Varying Ov and Mo Clusters

In an example, plasma treatment was used to prepare CuWO₄ HNS with different concentrations of Ov. CuWO₄ hollow nanosphere powder was put into quartz boats and transferred into the plasma chamber, where it was exposed to plasma. The CuWO₄ hollow nanosphere powder was exposed to O₂ plasma for 300 seconds with RF power of 200 W, resulting in CuWO₄ hollow nanospheres with low concentration of O_v (L—CuW). In another example, the CuWO₄ hollow nanosphere powder was exposed to Ar plasma for 300 seconds with a gas flow of 10 sccm, resulting in CuWO₄hollow nanospheres with high concentration of O_v (H—CuW).

After plasma treatment, the L—CuW, or H—CuW, were subjected to vigorous magnetic stirring at constant temperature (80° C.) in a 10 mL solution including 20 mg ammonium paramolybdate ((NH₄)₆Mo₇O₂₄·4H₂O). The resulting powders were then washed with deionized water and ethanol, and then dried at 80° C. for 12 h and annealed at 100° C. for 1 h in H₂/Ar atmosphere. The Mo-treated L—CuW and H—CuW are referred herein as Mo/L—CuW and Mo/H—CuW, respectively.

Preparation of Electrodes Including CuWO₄ Hollow Nanospheres

In an example, an electrode including the CuWO₄ hollow nanosphere catalyst described above was prepared as follows:

• • cleaning a carbon cloth (CC, 1 cm×1 cm) with acetone, ethanol, and deionized water several times;
  • preparing a solution including 40 μL of 5 wt. % Nafion and 960 μL of deionized water/ethanol (v/v=1:3);
  • dispersing 5 mg of the catalyst in the solution;
  • sonicating the solution in an ultrasonic cleaning machine for 1 h to form a homogeneous catalyst ink; and
  • applying 50 μL of the catalyst ink on the clean carbon cloth and drying at room temperature.

All the electrochemical measurements were conducted on Gamry G300 potential station in an H-type electrolytic cell separated by a Nafion 117 membrane (DuPont) in a typical standard three-electrode system, in which the above as-prepared catalyst/CC act as a working electrode, and a piece of CC and Ag/AgCl (Saturated KCl electrolyte) represent the counter and reference electrode, respectively.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprise” (and any form of comprise, such as “comprises” and “comprising”), “have” (and any form of have, such as “has” and “having”), “include” (and any form of include, such as “includes” and “including”), “contain” (and any form contain, such as “contains” and “containing”), and any other grammatical variant thereof, are open-ended linking verbs. As a result, a method or article that “comprises”, “has”, “includes” or “contains” one or more steps or elements possesses those one or more steps or elements, but is not limited to possessing only those one or more steps or elements. Likewise, a step of a method or an element of an article that “comprises”, “has”, “includes” or “contains” one or more features possesses those one or more features, but is not limited to possessing only those one or more features.

Terms like “obtainable” or “definable” and “obtained” or “defined” are used interchangeably. This, for example, means that, unless the context clearly dictates otherwise, the term “obtained” does not mean to indicate that, for example, an embodiment must be obtained by, for example, the sequence of steps following the term “obtained” though such a limited understanding is always included by the terms “obtained” or “defined” as a preferred embodiment.

Approximating language, as used herein throughout disclosure, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about” or “substantially,” is not limited to the precise value specified. For example, these terms can refer to an amount that is within ±10% of the recited value, an amount that is within ±5% of the recited value, less than or equal to ±2%, an amount that is within ±1% of the recited value, an amount that is within ±0.5% of the recited value, an amount that is within ±0.2% of the recited value, an amount that is within ±0.1% of the recited value, or an amount that is within ±0.05% of the recited value. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value.

All publications cited in this specification are herein incorporated by reference as if each individual publication were specifically and individually indicated to be incorporated by reference herein as though fully set forth.

Subject matter incorporated by reference is not considered to be an alternative to any claim limitations, unless otherwise explicitly indicated.

Where one or more ranges are referred to throughout this specification, each range is intended to be a shorthand format for presenting information, where the range is understood to encompass each discrete point within the range as if the same were fully set forth herein.

While several aspects and embodiments of the present disclosure have been described and depicted herein, alternative aspects and embodiments may be affected by persons having ordinary skill in the art to accomplish the same objectives. Accordingly, this disclosure and the appended claims are intended to cover all such further and alternative aspects and embodiments as fall within the true spirit and scope of the present disclosure.

Claims

1. A method of making a catalyst, comprising: forming a CuWO4 hollow nanosphere by a hydrothermal process followed by a thermal treatment;plasma-treating the CuWO4 hollow nanosphere with a plasma to introduce oxygen vacancies; andintroducing Mo clusters adjacent to the oxygen vacancies;wherein the hydrothermal process comprises mixing a copper source, a tungsten source, and an adjuvant to form a solution, heating the solution to form a precursor precipitate, and drying the precursor precipitate; andwherein the thermal treatment comprises annealing the precursor precipitate to form a CuWO4 hollow nanosphere.

2. The method of claim 1, wherein the copper source is a water-soluble copper salt, the tungsten source is a water-soluble tungsten salt, and the adjuvant is sodium citrate dihydrate (C6H5O7Na3·2H2O).

3. The method of claim 1, wherein a ratio of copper source:adjuvant:tungsten source is in a range of about 1:1:0.5 to about 1:1:1.

4. The method of claim 1, wherein heating is performed at a temperature in a range of about 160° C. to about 180° C. for about 22 hours to about 26 hours.

5. The method of claim 1, wherein annealing the precursor precipitate is performed at a temperature in a range of about 350° C. to about 450° C. for about 1 hour to about 2 hours.

6. The method of claim 1, wherein plasma-treating comprises one or both of (i) exposing the CuWO4 hollow nanosphere to a plasma for about 300 seconds to about 600 seconds; and (ii) exposing the CuWO4 hollow nanosphere to a plasma with RF power about 200 W and gas flow in a range of about 10 sccm to about 20 sccm.

7. The method of claim 1, wherein the plasma is Ar or N2 and the CuWO4 hollow nanosphere has a high concentration of oxygen vacancies.

8. The method of claim 1, wherein the plasma is O2 and the CuWO4 hollow nanosphere has a low concentration of oxygen vacancies.

9. The method of claim 1, wherein introducing Mo clusters comprises (i) mixing polyoxomolybdate and the plasma-treated CuWO4 hollow nanosphere in water and evaporating the solution to obtain a powder comprising the plasma-treated CuWO4 hollow nanosphere with Mo cluster inclusions; and(ii) annealing the powder at a temperature in a range of about 80° C. to about 100° C. for about 1 hour to about 2 hours in H2/Ar atmosphere.

10. A catalyst comprising: a CuWO4 hollow nanosphere;asymmetric oxygen vacancies within the CuWO4 hollow nanosphere; andMo clusters within the CuWO4 hollow nanosphere, wherein the Mo clusters are adjacent to the asymmetric oxygen vacancies.

11. The catalyst of claim 10, wherein the concentration of asymmetric oxygen vacancies is low.

12. The catalyst of claim 10, wherein the concentration of asymmetric oxygen vacancies is high.

13. The catalyst of claim 10, wherein the diameter of the CuWO4 hollow nanosphere is in a range of about 300 nm to about 450 nm.

14. The catalyst of claim 10, wherein the catalyst is made by a method comprising: forming a CuWO4 hollow nanosphere by a hydrothermal process followed by a thermal treatment;plasma-treating the CuWO4 hollow nanosphere with a plasma to introduce oxygen vacancies; andintroducing Mo clusters adjacent to the oxygen vacancies;wherein the hydrothermal process comprises mixing a copper source, a tungsten source, and an adjuvant to form a solution, heating the solution to form a precursor precipitate, and drying the precursor precipitate; andwherein the thermal treatment comprises annealing the precursor precipitate to form a CuWO4 hollow nanosphere.

15. A method of preparing an electrode for nitrate reduction, the method comprising: forming a solution comprising a plurality of CuWO4 hollow nanospheres and Nafion;sonicating the solution to form a homogeneous catalyst ink; andapplying the catalyst ink to a support to obtain the electrode.

16. The method of claim 15, wherein the solution further comprises one or more of ethanol, acetone, or water, or any combination thereof.

17. The method of claim 15, wherein the electrode exhibits one or both of (i) a high NH3 Faradaic efficiency of about 94.60±3.75%, and (ii) a yield rate of about 5.84±0.45 mg h−1 mgcat.−1 at −0.7 V versus RHE.

18. The method of claim 15, wherein the CuWO4 hollow nanospheres comprise: CuWO4 , wherein each O atom is linked to at least one W atom and at least one Cu atom;asymmetric oxygen vacancies; andMo clusters, wherein the Mo clusters are adjacent to the asymmetric oxygen vacancies.

19. The method of claim 18, wherein the concentration of asymmetric oxygen vacancies is low.

20. The method of claim 18, wherein the concentration of asymmetric oxygen vacancies is high.