CN115584527B - Preparation method and application of mesoporous palladium-copper nanocatalyst for nitrate reduction to produce ammonia - Google Patents

Abstract

The invention discloses a method for preparing a mesoporous palladium-copper nanocatalyst for nitrate reduction to produce ammonia, comprising the following steps: S1, dissolving dioctadecyl dimethyl ammonium chloride in a co-solvent of water and ethanol; S2, adjusting the pH of the reaction solution to 7-8 using sodium hydroxide; S3, sequentially adding chloropalladium copper nitrate solution to the reaction solution; S4, adding ascorbic acid for reduction; S5, removing surfactant by centrifugal washing to obtain a mesoporous palladium-copper nanocatalyst. The invention provides a method for preparing a mesoporous palladium-copper nanocatalyst for nitrate reduction to produce ammonia, by synthesizing a mesoporous palladium-copper nanocatalyst, not only the utilization rate of precious metals is improved, but also a mesoporous structure with unique functionality is well introduced, and the mesoporous structure improves the activity of electro-reduction of nitrate to synthesize ammonia, especially for ammonia selectivity. The present invention also provides an application of a mesoporous palladium-copper nanocatalyst for nitrate reduction to produce ammonia.

Description

A preparation method and application of a mesoporous palladium-copper nanocatalyst for nitrate reduction to produce ammonia

Technical Field

The invention relates to the technical field of catalysts, and in particular to a method for preparing a mesoporous palladium-copper nanocatalyst for producing ammonia by reducing nitrate.

Background Art

Ammonia (NH ₃ ) is one of the most basic chemical raw materials in the world. It is not only an indispensable chemical raw material for fertilizers, pharmaceuticals, dyes, etc., but also considered to be an ideal green carbon-free fuel for low-carbon footprint energy technologies. At the same time, due to its high energy density (4.3kW h kg-1), it is also one of the most promising energy carriers. As a promising hydrogen carrier, unlike liquid hydrogen that needs to be stored under more stringent conditions, NH ₃ can be easily liquefied by increasing the pressure to ≈10 bar at room temperature or cooling to -33°C at atmospheric pressure. In fact, once direct ammonia fuel cells reach a higher level of maturity, ammonia is expected to become the main carbon-free fuel for ships and heavy transport vehicles. Therefore, it can be foreseen that the demand for NH ₃ will increase.

At present, industrial-scale NH ₃ synthesis still relies heavily on the Haber-Bosch process, which requires harsh operating conditions, including high temperature (400-500℃), high pressure (150-300atm), heterogeneous iron-based catalysts and high-purity hydrogen produced by natural gas reforming. Although this traditional process has promoted the development of synthetic ammonia, due to its huge annual output and energy- and emission-intensive process, the NH ₃ synthesis industry has accounted for 1-2% of the world's energy supply and 1% of the total global energy carbon dioxide emissions. Therefore, in order to meet the two-degree Celsius (2DS) target of COP21 for synthetic ammonia, alternative sustainable strategies compatible with renewable energy that can meet the growing demand are urgently needed to replace the traditional Haber-Bosch process.

Electrocatalytic methods have emerged as a clean energy route for decentralized ammonia production at room temperature and at various infrastructure scales, potentially driven by locally generated renewable energy. The electrochemical nitrogen reduction reaction (NRR) has recently attracted extensive research interest. It achieves electrocatalytic N 2 to NH ₃ conversion under mild conditions and high compatibility with renewable electricity, using readily available nitrogen and water as nitrogen and proton sources, respectively. However, the inherent nonpolarity of nitrogen, the high dissociation energy of the N≡N bond (945 kJ mol-1), the low water solubility (0.66 mmol L- ₁ at 25 °C and 1 bar), and the competitive hydrogen evolution reaction (HER) during the reaction process make the selectivity of NRR ammonia synthesis significantly insufficient, and the ammonia yield (<200 μg h-1 mgcat-1) is even two to three orders of magnitude lower than that of the Haber-Bosch process. In contrast, NO _{3 -} is particularly attractive, showing a relatively lower N=O bond dissociation energy (204 kJ mol-1) and higher solubility (3.76 mol L-1) compared to nitrogen. Theoretically, the reaction barrier of the electrocatalytic nitrate reduction reaction (NITRR) occurring at the solid-liquid interface is lower than that of the solid-gas-liquid interface of NRR. Therefore, from the perspective of structural properties, electrocatalytic NITRR is easier to proceed. At the same time, NO ₃ -, as one of the most widespread water pollutants in the world, is widely present in surface water, groundwater and nuclear waste, which is harmful to the environment and human health. Therefore, from the comprehensive energy and environmental aspects, it is of greater significance to convert waste nitric acid aqueous solution into ammonia value-added products.

The process of NO ₃ -converting to NH ₃ involves a nine-proton and eight-electron transfer reaction (NO ₃ -+6H2O+8e-→NH ₃ +9OH-), thus significantly reducing the overall kinetic rate. In addition, the reaction potential of electrocatalytic nitrate reduction to produce ammonia (1.20V vs. reversible hydrogen electrode (RHE)) is slightly lower than that of NO ₃ -converting to N ₂ with a five-electron transfer process (1.25V vs. reversible hydrogen electrode (RHE)). This makes NO ₃ -reduction mostly tend to be a five-electron process to reduce to N ₂ instead of the desired NH _3. More importantly, the actual potential of NO ₃ -converting to NH ₃ is usually lower than the hydrogen evolution reaction (HER) potential (0V vs. reversible hydrogen electrode (RHE)), which also causes the production of hydrogen and consumption of electron donors, ultimately leading to low Faradaic efficiency. In addition, the complex products of NITRR may include NO ₂ -, N ₂ and NH ₃ , which also poses a major challenge to the goal of high-selective synthesis.

Many transition metals or noble metals (such as Pt, Pd, Ru, Rh, Ag, and Cu) have been used to design NITRR electrocatalysts. However, their activities are relatively low due to the inefficient NO ₃ - adsorption and electron injection into the π* antibonding orbital caused by the planar resonant D3h configuration and the high energy of the lowest unoccupied orbital of NO3-. Although the traditional alloying strategy uses promoters such as Sn, Cu, and Ge to adsorb and activate NO ₃ - and initiate the reaction, and noble metals are used to control the reaction selectivity, the products usually show a wide distribution and exhibit low ammonia selectivity. This is because the strong competition between different nitrogen-containing byproducts and HER makes the reaction complicated and further reduces the Faradaic efficiency of ammonia. Therefore, it is very desirable to design catalysts that can selectively reduce NO ₃ - to NH ₃ and suppress N≡N bond formation and HER.

Summary of the invention

The purpose of the present invention is to provide a method for preparing a mesoporous palladium-copper nanocatalyst for nitrate reduction to produce ammonia. By synthesizing the mesoporous palladium-copper nanocatalyst, not only the utilization rate of precious metals is improved, but also a mesoporous structure with unique functionality is well introduced. The mesoporous structure improves the activity of electro-reduction of nitrate to synthesize ammonia, especially significantly improving the ammonia selectivity.

The technical scheme adopted by the preparation method and application of a mesoporous palladium-copper nanocatalyst for nitrate reduction to produce ammonia disclosed in the present invention is:

A method for preparing a mesoporous palladium-copper nanocatalyst for nitrate reduction to produce ammonia comprises the following steps:

S1, dissolving dioctadecyl dimethyl ammonium chloride in a co-solvent of water and ethanol;

S2, adjusting the pH of the reaction solution in step S1 to 7-8 using sodium hydroxide;

S3, adding chloropalladic acid and copper nitrate solution to the reaction solution obtained in step S2 in sequence;

S4, adding ascorbic acid to the reaction solution obtained in step S3 for reduction;

S5, after the reaction of step S4 is completed, the surfactant is removed by centrifugal washing to obtain a mesoporous palladium-copper nanocatalyst.

As a preferred embodiment, in step S1, dioctadecyl dimethyl ammonium chloride is dissolved in deionized water at 73-78° C., and after complete dissolution, anhydrous ethanol is added and mixed evenly, and the volume ratio of dioctadecyl dimethyl ammonium chloride, anhydrous ethanol and deionized water is 1.5:1:4.

As a preferred embodiment, in step S2, after the reaction solution is cooled, the concentration of the added sodium hydroxide is 0.1 mol/L, and the volume ratio of dioctadecyl dimethyl ammonium chloride to sodium hydroxide is 15:1.

As a preferred embodiment, in step S3, the concentrations of chloropalladic acid and copper nitrate solution are both 0.01 mol/L, and the volume ratio of dioctadecyldimethylammonium chloride, chloropalladic acid and copper nitrate solution is 25:4:3.

As a preferred embodiment, in step S4, after the reaction solution is allowed to stand at room temperature for 0.4h-0.6h, 0.3mol/L ascorbic acid is added for reduction, the reaction time is 1-2h, and the volume ratio of dioctadecyldimethylammonium chloride to ascorbic acid is 3:1.

As a preferred embodiment, in step S5, the solvent used for centrifugal washing is a mixed solution of anhydrous ethanol and deionized water in a volume ratio of 3:1.

As a preferred solution, the mesoporous palladium-copper catalyst with a component ratio of Pd ₆₃ Cu ₃₇ is obtained in step S5.

An application of a mesoporous palladium-copper nanocatalyst for nitrate reduction to produce ammonia, comprising physically loading the above-mentioned mesoporous palladium-copper nanocatalyst and Vulcan XC-72 carbon black in a mass ratio of 1:1 and drop-coating the same on clean carbon paper as a cathode catalyst, with a catalyst loading of 0.1 mg/cm2, and applying the catalyst to electro-reduction of nitrate to synthesize ammonia after drying.

As a preferred solution, a three-electrode system is used, the mesoporous palladium-copper nanocatalyst is drop-coated on carbon paper, a clip electrode is used as the working electrode, a saturated silver/silver chloride electrode is used as the reference electrode, a platinum sheet electrode is used as the counter electrode, and a mixed solution of potassium hydroxide and potassium nitrate is used as the electrolyte.

As a preferred embodiment, the potassium hydroxide concentration in the electrolyte is 0.1 mol/L, the potassium nitrate concentration is 0.01 mol/L, and during the catalyst preparation process, ethanol: deionized water: naphthol with a volume ratio of 15:10:2 is used as the solvent, and the catalyst concentration is 5 mg/mL.

The beneficial effects of the preparation method of a mesoporous palladium-copper nanocatalyst for nitrate reduction to produce ammonia disclosed in the present invention are as follows: the structural and component advantages of the prepared mesoporous palladium-copper nanocatalyst can greatly improve the utilization rate of precious metals and greatly reduce the cost of raw materials; and the one-step synthesis method is simple, easy to operate, can be prepared in large quantities, and is suitable for large-scale industrial production. In addition, due to the mesoporous characteristics and adjustable components of the mesoporous palladium-copper nanocatalyst, favorable adsorption of nitrate can be achieved. Therefore, the invention brings better effects on the degradation and utilization of nitrate pollution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an X-ray diffraction pattern (XRD) of the mesoporous Pd ₆₃ Cu ₃₇ nanocatalyst prepared in Example 1 of the present invention and a comparative catalyst (Pd ₆₃ Cu ₃₇ nanoparticles (NPs), Pd mesoporous nanospheres (MSs), PdNPs).

FIG. 2 is a transmission electron microscope (TEM) image of the mesoporous Pd ₆₃ Cu ₃₇ nanocatalyst prepared in Example 1 of the present invention.

FIG3 is a linear sweep voltammetry (LSV) curve of the mesoporous Pd ₆₃ Cu ₃₇ nanocatalyst prepared in Example 1 of the present invention (with or without KNO ₃ ).

FIG4 is an LSV curve of the mesoporous Pd ₆₃ Cu ₃₇ nanocatalyst prepared in Example 1 of the present invention and a comparative catalyst (Pd ₆₃ Cu ₃₇ nanoparticles (NPs), Pd mesoporous nanospheres (MSs), PdNPs).

5 is a graph showing the electroreduction of nitrate to ammonia performance of the mesoporous Pd ₆₃ Cu ₃₇ nanocatalyst prepared in Example 1 of the present invention and comparative catalysts (Pd ₆₃ Cu ₃₇ nanoparticles (NPs), Pd mesoporous nanospheres (MSs), PdNPs) (electrolysis at different voltages for 2 hours, the electrolyte being 0.1 mol/LKOH+0.01 mol/LKNO ₃ ).

6 is a graph showing the energy efficiency (EE) of electroreduction of nitrate to ammonia by the mesoporous Pd ₆₃ Cu ₃₇ nanocatalyst prepared in Example 1 of the present invention and the comparative catalysts of Comparative Examples 1-3 (Pd ₆₃ Cu ₃₇ nanoparticles (NPs), Pd mesoporous nanospheres (MSs), PdNPs).

7 is a graph showing the cyclic stability performance of the mesoporous Pd ₆₃ Cu ₃₇ nanocatalyst prepared in Example 1 of the present invention and comparative catalysts (Pd ₆₃ Cu ₃₇ nanoparticles (NPs), Pd mesoporous nanospheres (MSs), PdNPs) (each catalyst was electrolyzed for 2 hours at the optimal voltage, and the electrolyte was 0.1 mol/L KOH+0.01 mol/L KNO ₃ ).

DETAILED DESCRIPTION

The present invention will be further described and illustrated below in conjunction with specific embodiments and accompanying drawings:

Mesoporous noble metal-based nanoparticles are a new type of nanostructured material, whose solid skeleton is surrounded by 2-50nm mesopores to form complete and uniform nanoparticles. Mesoporous metal materials have recently been considered as potential selective (electro)catalysts. There are three main structural advantages: (i) metal mesopores provide confined clamping sites and change the coordination/chemical adsorption properties of reactants, selectively promoting favorable products and inhibiting their competing reactions; (ii) mesopores provide good space for stabilizing unfavorable active sites and retaining reaction intermediates that selectively promote (electro)catalysis to high-value products; (iii) long-channel mesopores provide stable and active nanospaces to increase the retention time of deep (electro)catalytic multi-step reactions to generate selective products. Therefore, mesoporous noble metal-based materials are an important research direction for promoting the improvement of the performance (especially selectivity) of electroreduction of nitrates to synthesize ammonia. In view of this, the present invention is specially proposed.

A method for preparing a mesoporous palladium-copper nanocatalyst for nitrate reduction to produce ammonia comprises the following steps:

S1, dissolving dioctadecyl dimethyl ammonium chloride in a co-solvent of water and ethanol, dissolving dioctadecyl dimethyl ammonium chloride in deionized water at 73-78°C, adding anhydrous ethanol and mixing evenly after the dissolution is complete, and the volume ratio of dioctadecyl dimethyl ammonium chloride, anhydrous ethanol and deionized water is 1.5:1:4;

S2, adjusting the pH of the reaction solution of step S1 to 7-8 using sodium hydroxide, after the reaction solution is cooled, the concentration of the added sodium hydroxide is 0.1 mol/L, and the volume ratio of dioctadecyl dimethyl ammonium chloride to sodium hydroxide is 15:1;

S3, adding chloropalladic acid and copper nitrate solution to the reaction solution obtained in step S2 in sequence, wherein the concentrations of chloropalladic acid and copper nitrate solution are both 0.01 mol/L, and the volume ratio of dioctadecyldimethylammonium chloride, chloropalladic acid, and copper nitrate solution is 25:4:3;

S4, adding ascorbic acid to the reaction solution obtained in step S3 for reduction, after the reaction solution is allowed to stand at room temperature for 0.4 h to 0.6 h, adding 0.3 mol/L ascorbic acid for reduction, the reaction time is 1 to 2 h, and the volume ratio of dioctadecyl dimethyl ammonium chloride to ascorbic acid is 3:1;

S5, after the reaction of step S4 is completed, the surfactant is removed by centrifugal washing to obtain a mesoporous palladium-copper nanocatalyst, the solvent used for centrifugal washing is a mixed solution of anhydrous ethanol and deionized water in a volume ratio of 3:1, and the obtained is a mesoporous palladium-copper catalyst with a component ratio of Pd ₆₃ Cu ₃₇ .

An application of a mesoporous palladium-copper nanocatalyst for nitrate reduction to produce ammonia, comprising physically loading the above-mentioned mesoporous palladium-copper nanocatalyst and Vulcan XC-72 carbon black in a mass ratio of 1:1 and drop-coating the same on clean carbon paper as a cathode catalyst, with a catalyst loading of 0.1 mg/cm2, and applying the catalyst to electro-reduction of nitrate to synthesize ammonia after drying.

A three-electrode system was used, and the mesoporous palladium-copper nanocatalyst was drop-coated on carbon paper. A clip electrode was used as the working electrode, a saturated silver/silver chloride electrode was used as the reference electrode, a platinum sheet electrode was used as the counter electrode, and a mixed solution of potassium hydroxide and potassium nitrate was used as the electrolyte.

The potassium hydroxide concentration in the electrolyte is 0.1 mol/L, the potassium nitrate concentration is 0.01 mol/L, and during the catalyst preparation process, ethanol: deionized water: naphthol with a volume ratio of 15:10:2 is used as the solvent, and the catalyst concentration is 5 mg/mL.

Example 1

Preparation of mesoporous Pd63Cu37 nanocatalyst:

3 mg of dioctadecyl dimethyl ammonium chloride (DODAC) was dissolved in 8 mL of deionized water at 75°C, and after the solution was completely dissolved, 2 mL of anhydrous ethanol was added to mix evenly; after the reaction solution was cooled, 0.2 mL of a 0.1 mol/L sodium hydroxide solution was added to adjust the pH of the above reaction solution; 0.48 mL of a 0.01 mol/L chloropalladic acid and 0.36 mL of a 0.01 mol/L copper nitrate solution were sequentially added to the reaction solution obtained in the previous step; after the reaction solution was allowed to stand at room temperature for 0.5 h, 1 mL of 0.3 mol/L ascorbic acid was added for reduction, and the reaction time was 1-2 h; after the reaction was completed, a mixed solution of anhydrous ethanol and deionized water in a volume ratio of 3:1 was used for centrifugal washing to remove the surfactant, and a mesoporous Pd ₆₃ Cu ₃₇ catalyst with an optimal component ratio was obtained.

Example 2

Preparation of Mesoporous Pd-Cu Electrode

Weigh 1 mg of the mesoporous Pd ₆₃ Cu ₃₇ nanocatalyst prepared in the embodiment, mix and stir (more than 3 hours) with 1 mg of Vulcan XC-72 carbon black solution for physical loading for catalysis. Mix 2 mg of the mesoporous Pd ₆₃ Cu ₃₇ nanocatalyst loaded with carbon black with 400 μL of deionized water, 520 μL of anhydrous ethanol and 80 μL of naphthol solution, and ultrasonicate the mixed solution for 0.5 hours to obtain a 1 mg/mL mesoporous Pd ₆₃ Cu ₃₇ nanocatalyst solution. Use a pipette to draw 50 μL of the catalyst solution and evenly drop it on a 1 cm×2 cm clean carbon paper to make the catalyst loading amount 0.1 mg/cm-2. After drying, use it with the clip electrode as a mesoporous palladium copper electrode.

Example 3

As a comparative example of Example 2, comparative catalysts (Pd ₆₃ Cu ₃₇ nanoparticles (NPs), Pd mesoporous nanospheres (MSs), Pd NPs) were synthesized for comparison of catalytic properties.

Example 4

Electrocatalytic reduction of nitrate to synthesize ammonia

An electrochemical workstation (Chenhua 660e) was used to carry out the electrocatalytic reduction of nitrate to synthesize ammonia. The test used a three-electrode system, with a mesoporous palladium copper electrode as the working electrode, a saturated silver/silver chloride electrode as the reference electrode, and a 1cm×2cm platinum sheet electrode as the counter electrode. The electrolytic cell used an H-type electrolytic cell with a mixed solution of 0.1mol/L potassium hydroxide and 0.01mol/L potassium nitrate as the electrolyte. Before the electrochemical test and during the reduction process, argon gas with a purity of more than 99.9% needs to be passed to saturation. The test temperature is 25°C.

Example 5

As a comparative example of Example 4, in order to qualitatively test whether the catalyst has nitrate reduction activity, the process of Example 4 was repeated, except that no nitrate (pure KOH) was added to the electrolyte.

Comparative Example 1

Preparation of Pd ₆₃ Cu ₃₇ Nanoparticles (NPs)

Under room temperature conditions, 0.48 mL of 0.01 mol/L chloropalladic acid and 0.36 mL of 0.01 mol/L copper nitrate solution were added to 10 mL of deionized water in sequence; 0.28 mL of 1 mg/mL Vulcan XC-72 carbon black solution was added and shaken well, and then 1 mL of 0.3 mol/L ascorbic acid was added for reduction, and the reaction time was 1-2 h; after the reaction was completed, a mixed solution of anhydrous ethanol and deionized water in a volume ratio of 3:1 was used for centrifugal washing to obtain Pd ₆₃ Cu ₃₇ NPs with the same component ratio.

Comparative Example 2

Preparation of Pd mesoporous nanospheres (MSs)

3 mg DODAC was dissolved in 8 mL deionized water at 75 °C. After complete dissolution, 2 mL of anhydrous ethanol was added to mix evenly. After the reaction solution was cooled, 0.2 mL of 0.1 mol/L sodium hydroxide solution was added to adjust the pH of the reaction solution. 0.48 mL of 0.01 mol/L chloropalladic acid was added to the reaction solution obtained in the previous step. After the reaction solution was allowed to stand at room temperature for 0.5 h, 1 mL of 0.3 mol/L ascorbic acid was added for reduction. The reaction time was 1-2 h. After the reaction was completed, a mixed solution of anhydrous ethanol and deionized water in a volume ratio of 3:1 was used for centrifugal washing to remove the surfactant to obtain PdMSs.

Comparative Example 3

Preparation of Pd nanoparticles (NPs)

At room temperature, weigh 1 mg of palladium black, add 1 mL of 1 mg/mL Vulcan XC-72 carbon black solution and mix and stir (for more than 3 hours) for physical loading, and then use a mixed solution of anhydrous ethanol and deionized water in a volume ratio of 3:1 to obtain PdNPs by centrifugation.

In the test of electrocatalytic nitrate reduction to synthesize ammonia, the linear sweep voltammetry (LSV) is used to record the current density (j) within the voltage range, which can be used to characterize the electrochemical behavior within the voltage range, wherein the LSV scan rate is 10 mV/s, and the scanning voltage range is -0.6-0.2 V relative to the standard hydrogen electrode; the constant voltage method can be used to record the current-time (I-t) curve under the applied potential, which is used to record the charge (Q) to calculate the Faraday efficiency (FE); the voltage range applied in the embodiment of the present application is -0.05--0.4 V relative to the standard hydrogen electrode; the ammonia yield (NH3yieldrate) after the nitrate reduction reaction is tested, using the classic indophenol blue colorimetry, and the ultraviolet spectrophotometry is used for quantitative analysis.

The Faradaic efficiency of ammonia is calculated as follows:

FE(NH ₃ )=(8×F×C _NH3 ×V10 ^-6 )/(17×Q)×100%

The ammonia yield is calculated as follows:

Yieldrate(NH ₃ )=(C _NH3 ×V)/(t×m)

Where C _NH3 is the tested NH ₃ concentration (μg/mL); V is the volume of the electrolyte (30mL); t is the electrolysis time (2h); A is the geometric area of the electrode (0.5cm-2); F is the Faraday efficiency (96485C/mol); Q(C) is the total charge passing through the electrode, which is the result of the integration of the I-t curve.

The energy efficiency (EE) of ammonia is calculated as follows:

Energyefficiency(NH ₃ )=(1.23-E _NH30 )×FE(NH ₃ )/(1.23-E)×100%

_ENH30 is the equilibrium potential (0.70 V) for the electroreduction of nitrate to synthesize ammonia in alkaline medium; FE(NH ₃ ) is the Faradaic efficiency of NH ₃ ; 1.23 V is the equilibrium potential for water oxidation (i.e., assuming that the water oxidation overpotential is zero); E is the applied potential for the synthesis of NH ₃ (relative to the reversible hydrogen electrode).

Catalyst structure characterization and electro-reduction nitrate ammonia synthesis performance analysis of the embodiment:

Figure 1 is an X-ray diffraction pattern (XRD) of the mesoporous Pd ₆₃ Cu ₃₇ nanocatalyst prepared in Example 1 of the present invention and the comparative catalysts (Pd ₆₃ Cu ₃₇ nanoparticles (NPs), Pd mesoporous nanospheres (MSs), PdNPs) of Comparative Examples 1-3. The figure shows that Pd ₆₃ Cu ₃₇ MSs is face-centered cubic (fcc), and the positive shifts of all diffraction peaks indicate that Cu atoms with smaller atomic radius are alloyed in the form of substitution, shortening the lattice spacing of Pd-Pd bonds.

Figure 2 is a transmission electron microscope (TEM) image of the mesoporous Pd ₆₃ Cu ₃₇ nanocatalyst prepared in Example 1 of the present invention. The image shows that the catalyst is a highly uniformly dispersed mesoporous nanosphere with an average diameter of 45nm (Figure 2a). Three-dimensional radial and cylindrical open mesopores are distributed throughout the particles, with an average mesopore size of 3.7nm and a skeleton thickness of 4.1nm.

Figure 3 is the LSV curve of the mesoporous Pd ₆₃ Cu ₃₇ nanocatalyst prepared in Example 1 of the present invention in the electrochemical nitrate reduction test. The figure shows that compared with the electrolyte without nitrate (0.1 mol/L KOH), the nitrate reduction current density under the electrolyte with a mixed solution of 0.1 mol/L potassium hydroxide and 0.01 mol/L potassium nitrate is significantly increased, indicating that the mesoporous Pd ₆₃ Cu ₃₇ nanocatalyst has electrocatalytic nitrate reduction activity.

FIG4 is an LSV curve of the mesoporous Pd ₆₃ Cu ₃₇ nanocatalyst prepared in Example 1 of the present invention and the comparative catalysts (Pd ₆₃ Cu ₃₇ nanoparticles (NPs), Pd mesoporous nanospheres (MSs), PdNPs) of Comparative Examples 1-3. The figure shows that the nitrate reduction current density of the mesoporous Pd ₆₃ Cu ₃₇ nanocatalyst is the largest, indicating that the mesoporous Pd ₆₃ Cu ₃₇ nanocatalyst has the best electrocatalytic nitrate reduction activity.

FIG5 is a performance diagram of ammonia production and Faraday efficiency of electro-reduction of nitrate to synthesize ammonia by the mesoporous Pd ₆₃ Cu ₃₇ nanocatalyst prepared in Example 1 of the present invention and the comparative catalysts (Pd ₆₃ Cu ₃₇ nanoparticles (NPs), Pd mesoporous nanospheres (MSs), PdNPs) of Comparative Examples 1-3. The figure shows that the mesoporous Pd ₆₃ Cu ₃₇ nanocatalyst achieves the best Faraday efficiency of 85% at a voltage of -0.25V relative to the standard hydrogen electrode, and the corresponding ammonia production is 3058μgh-1mg-1. The performance is better than the comparative catalysts Pd ₆₃ Cu ₃₇ NPs (58%), PdMSs (72%) and PdNPs (49%) under the optimal conditions.

FIG6 is a graph showing the energy efficiency (EE) of electroreduction of nitrate to synthesize ammonia by the mesoporous Pd ₆₃ Cu ₃₇ nanocatalyst prepared in Example 1 of the present invention and the comparative catalysts (Pd ₆₃ Cu ₃₇ nanoparticles (NPs), Pd mesoporous nanospheres (MSs), PdNPs) of Comparative Examples 1-3. The graph shows that the Faradaic efficiency of the mesoporous Pd ₆₃ Cu ₃₇ nanocatalyst at a voltage of -0.25 V relative to the standard hydrogen electrode is 31%, which is much better than the comparative catalyst.

FIG7 is a graph showing the cyclic stability of electroreduction of nitrate to synthesize ammonia using the mesoporous Pd ₆₃ Cu ₃₇ nanocatalyst prepared in Example 1 of the present invention and the comparative catalysts (Pd ₆₃ Cu ₃₇ nanoparticles (NPs), Pd mesoporous nanospheres (MSs), PdNPs) of Comparative Examples 1-3. The graph shows that after six cycles, the mesoporous Pd ₆₃ Cu ₃₇ nanocatalyst has the best stability.

The present invention provides a method for preparing a mesoporous palladium-copper nanocatalyst for nitrate reduction to produce ammonia. The structural and component advantages of the prepared mesoporous palladium-copper nanocatalyst can greatly improve the utilization rate of precious metals and greatly reduce the cost of raw materials. The one-step synthesis method is simple and easy to operate, and can be prepared in large quantities, which is suitable for large-scale industrial production. In addition, due to the mesoporous characteristics and adjustable components of the mesoporous palladium-copper nanocatalyst, favorable adsorption of nitrate can be achieved. Therefore, the invention brings better effects on the degradation and utilization of nitrate pollution.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention, rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims (4)

1. A method for preparing a mesoporous palladium-copper nanocatalyst for nitrate reduction to produce ammonia, characterized in that it comprises the following steps: S1, dissolving dioctadecyl dimethyl ammonium chloride in a co-solvent of water and ethanol, dissolving dioctadecyl dimethyl ammonium chloride in deionized water at 73-78° C., adding anhydrous ethanol and mixing evenly after the dissolution is complete, wherein the volume ratio of dioctadecyl dimethyl ammonium chloride, anhydrous ethanol and deionized water is 1.5:1:4; S2, adjusting the pH of the reaction solution of step S1 to 7-8 using sodium hydroxide, after the reaction solution is cooled, the concentration of the added sodium hydroxide is 0.1 mol/L, and the volume ratio of dioctadecyl dimethyl ammonium chloride to sodium hydroxide is 15:1; S3, adding chloropalladic acid and copper nitrate solution to the reaction solution obtained in step S2 in sequence, wherein the concentrations of chloropalladic acid and copper nitrate solution are both 0.01 mol/L, and the volume ratio of dioctadecyldimethylammonium chloride, chloropalladic acid, and copper nitrate solution is 25:4:3; S4, adding ascorbic acid to the reaction solution obtained in step S3 for reduction, after the reaction solution is allowed to stand at room temperature for 0.4h-0.6h, adding 0.3mol/L ascorbic acid for reduction, the reaction time is 1-2h, and the volume ratio of dioctadecyl dimethyl ammonium chloride to ascorbic acid is 3:1; S5, after the reaction of step S4 is completed, the surfactant is removed by centrifugal washing to obtain a mesoporous palladium-copper nanocatalyst, the solvent used for centrifugal washing is a mixed solution of anhydrous ethanol and deionized water in a volume ratio of 3:1, and the obtained component ratio is Mesoporous palladium-copper catalyst. 2. An application of a mesoporous palladium-copper nanocatalyst for nitrate reduction to produce ammonia, characterized in that it includes a mesoporous palladium-copper nanocatalyst prepared by the preparation method of a mesoporous palladium-copper nanocatalyst for nitrate reduction to produce ammonia as described in claim 1 and VulcanXC-72 carbon black in a mass ratio of 1:1 and drop-coated it on clean carbon paper as a cathode catalyst, with a catalyst loading of 0.1 mg/cm2, and after drying, it is used for electro-reduction of nitrate to synthesize ammonia. 3. The use of a mesoporous palladium-copper nanocatalyst for nitrate reduction to produce ammonia as described in claim 2 is characterized in that a three-electrode system is adopted, the mesoporous palladium-copper nanocatalyst is drop-coated on carbon paper, a clip electrode is used as the working electrode, a saturated silver/silver chloride electrode is used as the reference electrode, a platinum sheet electrode is used as the counter electrode, and a mixed solution of potassium hydroxide and potassium nitrate is used as the electrolyte. 4. The use of a mesoporous palladium-copper nanocatalyst for nitrate reduction to produce ammonia as claimed in claim 3, characterized in that the potassium hydroxide concentration in the electrolyte is 0.1 mol/L, the potassium nitrate concentration is 0.01 mol/L, and in the catalyst preparation process, ethanol: deionized water: naphthol with a volume ratio of 15:10:2 is used as the solvent, and the catalyst concentration is 5 mg/mL.