A method for the electrocatalytic synthesis of urea from nitrates and carbon dioxide in kitchen wastewater.

Abstract

This invention relates to the fields of environmental engineering and electrochemistry, and discloses a method for the electrocatalytic synthesis of urea from nitrate and carbon dioxide in kitchen wastewater. The method includes pumping aerobic nitrification wastewater into the cathode chamber of a two-chamber electrochemical reactor, and pumping an electrolyte solution into the anolyte chamber to obtain the liquid phase system to be electrolyzed; introducing pure carbon dioxide gas for constant cathode reduction operation to obtain a slightly poisoned electrode system; applying a positive pulse potential to the slightly poisoned electrode system for in-situ cleaning and desorption to generate a copper oxide layer; and restoring the electrode system after in-situ cleaning and desorption to a constant cathode reduction state to reduce the copper oxide layer to activated zero-valent copper for a coupling reaction. This invention solves the problem of decreased catalytic activity caused by the poisoning of electrodes due to the adsorption of organic pollutants by applying a positive pulse potential to the slightly poisoned electrode system for in-situ cleaning and desorption.

Description

Technical Field

[0001] This invention relates to the fields of environmental engineering and electrochemical technology, specifically to a method for the electrocatalytic synthesis of urea from nitrates and carbon dioxide in kitchen waste biogas slurry. Background Technology

[0002] The goal is to synthesize urea by aerobic nitrification of anaerobic fermentation slurry from kitchen waste and then electrocatalytically coupling it with carbon dioxide. This is a way to realize the resource conversion and utilization of waste.

[0003] During the operation of a continuous flow electrochemical reaction system, organic pollutants in the kitchen waste biogas slurry can be adsorbed onto the electrode surface, leading to electrode poisoning and a decrease in catalytic activity. This makes it difficult for the continuous flow electrochemical reaction system to maintain long-term stable operation. During long-cycle electrolysis, the active state of the catalyst surface is difficult to maintain, and the efficiency of catalyzing the coupling reaction between nitrates and carbon dioxide gradually decreases. Simply relying on shutdown to replace electrodes or offline cleaning would increase operating costs and cause production interruptions.

[0004] In complex electrochemical reaction systems, the pH and salinity of the cathode liquid phase are dynamically changing, and this fluctuating bulk environment easily leads to scaling on the cation exchange membrane in the electrochemical reactor. The lack of matching control between the inlet pressure of gaseous carbon dioxide and the hydrostatic pressure of the liquid substrate affects the formation of the reaction interface on the electrode surface, limiting the continuous production of the target urea product.

[0005] Therefore, this invention proposes a method for the electrocatalytic synthesis of urea from nitrates and carbon dioxide in kitchen wastewater to address the shortcomings of existing technologies. Summary of the Invention

[0006] The purpose of this invention is to provide a method for the electrocatalytic synthesis of urea from nitrates and carbon dioxide in kitchen waste biogas slurry. This method solves the problems of decreased catalytic activity caused by the adsorption of organic pollutants from kitchen waste biogas slurry on the electrode surface during the electrocatalytic synthesis of urea from kitchen waste biogas slurry, as well as the difficulty in maintaining long-term stable operation of the continuous flow electrochemical reaction system and scaling of the cation exchange membrane.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] This invention provides a method for the electrocatalytic synthesis of urea from nitrates and carbon dioxide in kitchen waste biogas slurry, comprising the following steps:

[0009] The anaerobic fermentation slurry from kitchen waste is subjected to aerobic nitrification treatment and pre-treatment for calcium, magnesium and hardness removal to obtain aerobic nitrified slurry wastewater.

[0010] Copper catalyst powder is mixed with polytetrafluoroethylene hydrophobic binder, and the dry weight mass fraction of polytetrafluoroethylene in the mixture is controlled to be 5% to 15%. The mixture is coated on a copper foam substrate and pressed to obtain a gas diffusion electrode cathode containing polytetrafluoroethylene hydrophobic binder.

[0011] A gas diffusion electrode cathode containing a hydrophobic binder of polytetrafluoroethylene and a titanium-based ruthenium-iridium anode are assembled into a continuous flow electrochemical reaction system, and a cation exchange membrane is set between the gas diffusion electrode cathode containing a hydrophobic binder of polytetrafluoroethylene and the titanium-based ruthenium-iridium anode to obtain a dual-chamber electrochemical reactor with a cathode liquid chamber, an anode liquid chamber and an independent cathode gas chamber.

[0012] Aerobic nitrification wastewater is pumped into the cathode chamber of a dual-chamber electrochemical reactor, while an electrolyte solution is simultaneously pumped into the anolyte chamber. The pH of the cathode liquid phase is adjusted to 8.0 to 10.0 to obtain the liquid phase system to be electrolyzed.

[0013] Pure carbon dioxide gas is introduced into the independent cathode gas chamber of the dual-chamber electrochemical reactor. The inlet gas pressure is adjusted to be higher than the static pressure of the liquid column in the cathode liquid chamber of the dual-chamber electrochemical reactor, and the pressure difference is controlled between 2 kPa and 10 kPa. During this process, the pH value of the liquid phase system to be electrolyzed is maintained constant by adding acid-base regulators online, and the system salinity balance is maintained by continuous liquid phase feeding and discharging. The operating current density is controlled between 10 mA / cm2 and 50 mA / cm2 for constant cathode reduction operation for 30 min to 60 min, resulting in a solution system containing primary urea. At the same time, the surface of the gas diffusion electrode cathode containing polytetrafluoroethylene hydrophobic binder adsorbs organic pollutants from the kitchen waste biogas slurry and undergoes slight poisoning, resulting in a slightly poisoned electrode system.

[0014] In-situ cleaning and desorption were performed on the slightly poisoned electrode system by applying a positive pulse potential of 0.5V to 1.0V relative to the reversible hydrogen electrode for 1 to 5 seconds, and inducing slight anodic oxidation on the surface of the copper catalyst. This caused organic pollutants to be desorbed from the cathode surface of the gas diffusion electrode containing polytetrafluoroethylene hydrophobic binder, and a copper oxide layer was generated in situ, resulting in an electrode system that had completed in-situ cleaning and desorption.

[0015] The electrode system, after in-situ cleaning and desorption, is restored to a constant cathode reduction state with a controlled operating current density of 10 mA / cm2 to 50 mA / cm2. This allows the copper oxide layer to be reduced in situ to activated zero-valent copper rich in oxygen vacancies and with a high specific surface area, which continues to carry out the coupling reaction of nitrate and carbon dioxide, resulting in a continuous urea production system with restored catalytic activity.

[0016] An online gas analyzer is used to monitor the exhaust gas discharged from the continuous urea production system to restore catalytic activity in real time. Unreacted carbon dioxide gas is recycled after gas-liquid separation to obtain the target urea solution product.

[0017] During the constant cathode reduction operation phase, the surface of the gas diffusion electrode cathode containing polytetrafluoroethylene hydrophobic binder adsorbs organic pollutants from kitchen wastewater, resulting in slight poisoning. A positive pulse potential is applied to the slightly poisoned electrode system for in-situ cleaning and desorption, which simultaneously induces slight anodic oxidation on the surface of the copper catalyst, causing the organic pollutants to desorb from the surface of the gas diffusion electrode cathode containing polytetrafluoroethylene hydrophobic binder and generating a copper oxide layer in situ.

[0018] The electrode system, after in-situ cleaning and desorption, is restored to a constant cathodic reduction state, allowing the copper oxide layer to be reduced in situ to activated zero-valent copper rich in oxygen vacancies and with a high specific surface area, enabling continuous coupling reactions of nitrate and carbon dioxide, thus obtaining a urea continuous production system with restored catalytic activity. Applying a positive pulse potential relative to the reversible hydrogen electrode to the slightly poisoned electrode system is equivalent to applying an equivalent instantaneous reverse tank voltage step or a positive anode pulse current step, ensuring long-term stable operation of the continuous flow electrochemical reaction system and protecting the cation exchange membrane from scaling.

[0019] In summary, the present invention has at least one of the following beneficial technical effects:

[0020] 1. This invention solves the technical problem of reduced catalytic activity caused by poisoning of organic pollutants in the electrode adsorbed by the electrode system with slight poisoning by applying a positive pulse potential to the electrode system for in-situ cleaning and desorption, thereby desorbing organic pollutants from the cathode surface of the gas diffusion electrode containing polytetrafluoroethylene hydrophobic binder, and meeting the requirements for long-term stable operation of continuous flow electrochemical reaction system.

[0021] 2. This invention induces slight anodic oxidation on the surface of a copper catalyst by applying a positive pulse potential to a slightly poisoned electrode system, generating a copper oxide layer in situ. When the constant cathodic reduction state is restored, the copper oxide layer is reduced in situ to activated zero-valent copper rich in oxygen vacancies and with a high specific surface area, which promotes the coupling reaction between nitrate and carbon dioxide, resulting in a continuous urea production system with restored catalytic activity.

[0022] 3. This invention introduces pure carbon dioxide gas into the independent cathode gas chamber of a dual-chamber electrochemical reactor, adjusts the inlet gas pressure to make it higher than the static pressure of the liquid column in the cathode liquid chamber of the dual-chamber electrochemical reactor, combines online addition of acid and alkali regulators to maintain a constant pH value of the liquid phase system to be electrolyzed, and continuously refreshes the liquid phase to maintain the salinity balance of the system, protects the cation exchange membrane from scaling and realizes the conversion and utilization of waste resources. Attached Figure Description

[0023] Figure 1 This is a scatter plot of the urea generation rate fluctuating over time under continuous operation in Embodiment 1 of the present invention.

[0024] Figure 2 This is a broken line scatter plot comparing the Faraday efficiency distribution of the embodiments of the present invention with that of the comparative system;

[0025] Figure 3 This is a scatter plot showing the yield retention rate of the embodiment and comparative systems of the present invention as a function of operating time.

[0026] Figure 4 This is a graph showing the evolution of total copper concentration in the liquid phase effluent according to an embodiment of the present invention. Detailed Implementation

[0027] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] The positive pulse potential relative to the reversible hydrogen electrode described in the embodiments of the present invention is calibrated and controlled in the laboratory or small-scale test using a reference electrode. In the scale-up application of actual industrial continuous flow electrolyzers, due to the limited arrangement of the reference electrode, this operation is equivalent to applying an equivalent instantaneous reverse cell voltage step or a positive anode pulse current step to the electrode system.

[0029] Furthermore, to ensure the long-term stable operation of the continuous flow reactor and protect the ion exchange membrane from scaling, the actual anaerobic digestate substrate introduced into the reactor may be subject to conventional hardening or softening pretreatment steps, depending on the circumstances. Any engineering adaptations made by those skilled in the art based on the core mechanism of this invention do not depart from the scope of protection of this invention.

[0030] Examples 1-3:

[0031] Example 1:

[0032] This embodiment provides a method for the electrocatalytic synthesis of urea from nitrates and carbon dioxide in kitchen waste biogas slurry, including the following steps:

[0033] The anaerobic fermentation slurry from kitchen waste was subjected to aerobic nitrification treatment and pretreatment for calcium, magnesium and hardness removal to obtain aerobic nitrified slurry wastewater.

[0034] Copper catalyst powder was mixed with polytetrafluoroethylene hydrophobic binder, and the dry weight mass fraction of polytetrafluoroethylene in the mixture was controlled to be 10%. The mixture was coated on a copper foam substrate and pressed to obtain a gas diffusion electrode cathode containing 10% polytetrafluoroethylene hydrophobic binder.

[0035] A gas diffusion electrode cathode containing 10% polytetrafluoroethylene hydrophobic binder and a titanium-based ruthenium-iridium anode were assembled into a continuous flow electrochemical reaction system, and a cation exchange membrane was set between the anode and cathode to obtain an electrochemical reactor containing an anolyte liquid chamber, a cathode liquid chamber and an independent cathode gas chamber.

[0036] Aerobic nitrification wastewater was pumped into the cathode chamber of an electrochemical reactor containing a gas diffusion electrode cathode and a titanium-based ruthenium-iridium anode. At the same time, an electrolyte solution was pumped into the anode chamber, and the pH of the cathode liquid phase was adjusted to 9.0, resulting in a liquid phase system to be electrolyzed with a pH of 9.0.

[0037] Pure carbon dioxide gas is introduced into the independent cathode gas chamber of an electrochemical reactor containing a gas diffusion electrode cathode and a titanium-based ruthenium-iridium anode. The inlet gas pressure is adjusted to be slightly higher than the hydrostatic pressure of the liquid column in the cathode liquid chamber (controlling the pressure difference between 2-10 kPa). During this process, the pH value of the cathode liquid phase system is maintained constant at 9.0 by online addition of acid-base regulators, and continuous liquid phase inlet and outlet replenishment is used to maintain the salinity balance of the system. The operating current density is controlled at 30 mA / cm³. 2 After running a constant cathodic reduction for 45 minutes, a solution system containing primary urea was obtained. At the same time, the surface of the gas diffusion electrode cathode was slightly poisoned due to the adsorption of organic pollutants from the kitchen waste biogas slurry.

[0038] Apply a positive pulse potential of +0.8V relative to the reversible hydrogen electrode for 3 seconds to the slightly poisoned electrode system for in-situ cleaning and desorption, and simultaneously induce slight anodic oxidation on the surface of the copper catalyst, so that organic pollutants are desorbed from the cathode surface and a copper oxide layer is generated in situ on the copper catalyst surface.

[0039] The electrode system, after in-situ cleaning and desorption, was restored to a control operating current density of 30 mA / cm². 2 The constant cathode reduction state allows the copper oxide layer to be reduced in situ to activated zero-valent copper rich in oxygen vacancies and with a high specific surface area, so as to continuously carry out the coupling reaction of nitrate and carbon dioxide, thus obtaining a continuous urea production system with restored catalytic activity.

[0040] The tail gas discharged from the continuous urea production system to restore catalytic activity was detected in real time using an online gas analyzer. Unreacted carbon dioxide gas was recycled after gas-liquid separation. At the same time, the catholy liquid discharged from the system was collected to obtain the target urea solution product.

[0041] Example 2:

[0042] This embodiment provides a method for the electrocatalytic synthesis of urea from nitrates and carbon dioxide in kitchen waste biogas slurry, including the following steps:

[0043] The anaerobic fermentation slurry from kitchen waste was subjected to aerobic nitrification treatment and pretreatment for calcium, magnesium and hardness removal to obtain aerobic nitrified slurry wastewater.

[0044] Copper catalyst powder was mixed with polytetrafluoroethylene hydrophobic binder, and the dry weight mass fraction of polytetrafluoroethylene in the mixture was controlled to be 15%. The mixture was coated on a copper foam substrate and pressed to obtain a gas diffusion electrode cathode containing 15% polytetrafluoroethylene hydrophobic binder.

[0045] A gas diffusion electrode cathode containing 15% polytetrafluoroethylene hydrophobic binder and a titanium-based ruthenium-iridium anode were assembled into a continuous flow electrochemical reaction system, and a cation exchange membrane was set between the anode and cathode to obtain an electrochemical reactor containing an anolyte liquid chamber, a cathode liquid chamber and an independent cathode gas chamber.

[0046] Aerobic nitrification wastewater was pumped into the cathode chamber of an electrochemical reactor containing a gas diffusion electrode cathode and a titanium-based ruthenium-iridium anode. At the same time, an electrolyte solution was pumped into the anode chamber, and the pH of the cathode liquid phase was adjusted to 10.0, resulting in a liquid phase system to be electrolyzed with a pH of 10.0.

[0047] Pure carbon dioxide gas is introduced into the independent cathode gas chamber of an electrochemical reactor containing a gas diffusion electrode cathode and a titanium-based ruthenium-iridium anode. The inlet gas pressure is adjusted to be slightly higher than the hydrostatic pressure of the liquid column in the cathode liquid chamber (controlling the pressure difference between 2-10 kPa). During this process, the pH value of the cathode liquid phase system is maintained constant at 10.0 by online addition of acid-base adjusters, and continuous liquid phase inlet and outlet replenishment is used to maintain the salinity balance of the system. The operating current density is controlled at 50 mA / cm³. 2 After 60 minutes of constant cathodic reduction operation, a solution system containing primary urea was obtained. At the same time, the surface of the gas diffusion electrode cathode was slightly poisoned due to the adsorption of organic pollutants from the kitchen waste biogas slurry.

[0048] Apply a positive pulse potential of +1.0V relative to the reversible hydrogen electrode for 5 seconds to the slightly poisoned electrode system for in-situ cleaning and desorption, and simultaneously induce slight anodic oxidation on the surface of the copper catalyst, so that organic pollutants are desorbed from the cathode surface and a copper oxide layer is generated in situ on the copper catalyst surface.

[0049] The electrode system, after in-situ cleaning and desorption, was restored to a control operating current density of 50 mA / cm². 2The constant cathode reduction state allows the copper oxide layer to be reduced in situ to activated zero-valent copper rich in oxygen vacancies and with a high specific surface area, so as to continuously carry out the coupling reaction of nitrate and carbon dioxide, thus obtaining a continuous urea production system with restored catalytic activity.

[0050] The tail gas discharged from the continuous urea production system to restore catalytic activity was detected in real time using an online gas analyzer. Unreacted carbon dioxide gas was recycled after gas-liquid separation. At the same time, the catholy liquid discharged from the system was collected to obtain the target urea solution product.

[0051] Example 3:

[0052] This embodiment provides a method for the electrocatalytic synthesis of urea from nitrates and carbon dioxide in kitchen waste biogas slurry, including the following steps:

[0053] The anaerobic fermentation slurry from kitchen waste was subjected to aerobic nitrification treatment and pretreatment for calcium, magnesium and hardness removal to obtain aerobic nitrified slurry wastewater.

[0054] Copper catalyst powder was mixed with polytetrafluoroethylene hydrophobic binder, and the dry weight mass fraction of polytetrafluoroethylene in the mixture was controlled to be 5%. The mixture was coated on a copper foam substrate and pressed to obtain a gas diffusion electrode cathode containing 5% polytetrafluoroethylene hydrophobic binder.

[0055] A gas diffusion electrode cathode containing 5% polytetrafluoroethylene hydrophobic binder and a titanium-based ruthenium-iridium anode were assembled into a continuous flow electrochemical reaction system, and a cation exchange membrane was set between the anode and cathode to obtain an electrochemical reactor containing an anolyte liquid chamber, a cathode liquid chamber and an independent cathode gas chamber.

[0056] Aerobic nitrification wastewater was pumped into the cathode liquid chamber of an electrochemical reactor containing a gas diffusion electrode cathode and a titanium-based ruthenium-iridium anode. At the same time, an electrolyte solution was pumped into the anode liquid chamber, and the pH value of the cathode liquid phase was adjusted to 8.0, resulting in a liquid phase system to be electrolyzed with a pH value of 8.0.

[0057] Pure carbon dioxide gas is introduced into the independent cathode gas chamber of an electrochemical reactor containing a gas diffusion electrode cathode and a titanium-based ruthenium-iridium anode. The inlet gas pressure is adjusted to be slightly higher than the hydrostatic pressure of the liquid column in the cathode liquid chamber (controlling the pressure difference between 2-10 kPa). During this process, the pH value of the cathode liquid phase system is maintained constant at 8.0 by online addition of acid-base regulators, and continuous liquid phase inlet and outlet replenishment is used to maintain the salinity balance of the system. The operating current density is controlled at 10 mA / cm³. 2 After running a constant cathodic reduction for 30 minutes, a solution system containing primary urea was obtained. At the same time, the surface of the gas diffusion electrode cathode was slightly poisoned due to the adsorption of organic pollutants from the kitchen waste biogas slurry.

[0058] Apply a positive pulse potential of +0.5V relative to the reversible hydrogen electrode for 1 second to the slightly poisoned electrode system for in-situ cleaning and desorption, and simultaneously induce slight anodic oxidation on the surface of the copper catalyst, so that organic pollutants are desorbed from the cathode surface and a copper oxide layer is generated in situ on the copper catalyst surface.

[0059] The electrode system, after in-situ cleaning and desorption, was restored to a control operating current density of 10 mA / cm². 2 The constant cathode reduction state allows the copper oxide layer to be reduced in situ to activated zero-valent copper rich in oxygen vacancies and with a high specific surface area, so as to continuously carry out the coupling reaction of nitrate and carbon dioxide, thus obtaining a continuous urea production system with restored catalytic activity.

[0060] The tail gas discharged from the continuous urea production system to restore catalytic activity was detected in real time using an online gas analyzer. Unreacted carbon dioxide gas was recycled after gas-liquid separation. At the same time, the catholy liquid discharged from the system was collected to obtain the target urea solution product.

[0061] Comparative Examples 1-3:

[0062] Comparative Example 1:

[0063] Compared with Example 1, the difference is that the gas diffusion electrode cathode containing 10% polytetrafluoroethylene hydrophobic binder is replaced with a regular non-porous copper foil cathode, and the independent cathode gas chamber is eliminated. Pure carbon dioxide gas is directly introduced into the liquid phase system to be electrolyzed with a pH of 9.0 through the aeration head. All other aspects are the same.

[0064] Comparative Example 2:

[0065] Compared with Example 1, the difference is that no polytetrafluoroethylene hydrophobic binder is added during the cathode preparation process, resulting in a hydrophilic gas diffusion electrode cathode that does not contain polytetrafluoroethylene hydrophobic binder; all other aspects are the same.

[0066] Comparative Example 3:

[0067] The difference compared to Example 1 is that the operating current density was controlled at 30 mA / cm² in the electrolyte system with a pH of 9.0. 2 During the constant cathode reduction operation, no in-situ cleaning and desorption is performed by applying a positive pulse potential of +0.8V relative to the reversible hydrogen electrode for 3 seconds. The constant cathode reduction state is maintained and the operation continues. All other aspects are the same.

[0068] Test Examples 1-4:

[0069] Test Example 1:

[0070] The urea continuous production system with restored catalytic activity during the operation of Example 1 was selected as the experimental test object for macroscopic physicochemical verification.

[0071] When the system is running continuously, a fixed amount of liquid phase effluent samples are continuously extracted from the effluent line of the electrochemical reactor according to the preset time nodes.

[0072] The extracted liquid phase sample was subjected to a chemical colorimetric reaction using diacetyl oxime spectrophotometry, and the absorbance value was read. The mass concentration of urea was calculated based on the pre-drawn standard working curve. The absolute urea generation rate of the system at each time point was calculated by combining the known geometric projection area of ​​the electrode and the sampling time interval.

[0073] The online gas detector and analyzer connected to the exhaust pipeline is invoked synchronously to record and archive the instantaneous concentration values ​​of nitrogen oxides in the exhaust gas captured by the device probe at the corresponding liquid sampling time node.

[0074] Table 1. Measured data of product indicators at different running time points in Example 1

[0075] 2 14.52 1.1 10 14.28 0.8 24 14.81 1.4 48 13.95 0.9 72 14.43 1.2 96 13.79 0.7 120 14.16 1.0

[0076] The test results are as follows:

[0077] Figure 1 The scatter plots and connecting broken lines in the longitudinal direction represent the fluctuations in the urea production rate measured at different sampling time points within 120 hours in the system of Example 1.

[0078] Based on the data in Table 1 and Figure 1 It can be seen that during the 120-hour uninterrupted operation test, the product data curve of the system in Example 1 maintained a consistent lateral extension. The distribution of data points exhibited irregular, narrow-range fluctuations, consistent with the dynamic lag or acceleration response caused by the continuous entry of real anaerobic digestate containing unknown interfering impurities into the electrocatalytic system. After excluding the idealized flat data caused by the uniform composition of the laboratory artificial water mixing system, the product synthesis rate per unit area remained stable within the upper and lower bands of 14#imgpt1#.

[0079] Nitrate ions reach the gas-liquid-solid three-phase interface through a complex liquid flow field and directly capture non-hydrated carbon dioxide molecules diffusing from the cathode gas chamber within the reaction chamber constructed at the hydrophobic boundary, completing the crucial carbon-nitrogen atom coupling assembly. Intermittently applied positive potentials strip away large molecular clusters such as humic acid attached to the surface of the active sites, and the brief interface reconstruction cycle prevents the electrode passivation process caused by long-term electrolysis. The gas phase product parameters recorded at the online monitoring terminal consistently remain below the safety limit of 1.5 ppm, demonstrating that free nitrogen entering the reaction tank is effectively intercepted and directionally converted, preventing byproducts from escaping control and into the environment, thus fulfilling the pre-set requirements of the engineering route for exhaust emission tolerance limits.

[0080] Test Example 2:

[0081] Electrochemical reaction systems of Examples 1, 2, and 3, as well as Comparative Examples 1 and 2, at the same operating stage were selected as experimental subjects for performance evaluation and comparison.

[0082] Under the condition of keeping the initial influent substrate concentration and operating room temperature constant for each experimental subject, the electrocatalytic reduction process is started according to the preset constant current input parameters. After accumulating an equal amount of total charge, the power supply is cut off to stop operation.

[0083] The final liquid mixture discharged from the liquid chamber of each experimental subject was collected, and the total amount of urea generated in the liquid sample was determined by high-performance liquid chromatography. The corresponding urea Faraday efficiency value was calculated by combining the cumulative total charge flowing into the system.

[0084] The gas mixture escaping from the cathode side and liquid chamber during operation was collected simultaneously using a gas collection bag. The volume ratio of hydrogen was quantified by the thermal conductivity detector of a gas chromatograph, and the hydrogen evolution Faraday efficiency value of the charge consumed by the hydrogen evolution of each experimental object was calculated.

[0085] Table 2. Measured Faraday efficiency data of key products in the example and comparative systems.

[0086] Experimental subjects Urea Faraday efficiency (%) Hydrogen evolution Faraday efficiency (%) Example 1 22.4 11.7 Example 2 18.3 14.8 Example 3 20.1 9.5 Comparative Example 1 0.8 81.2 Comparative Example 2 5.7 64.9

[0087] The test results are as follows:

[0088] Figure 2 This is a scatter plot comparing the Faraday efficiency distribution of the embodiments and comparative examples of the present invention. The horizontal axis of the plot represents the test objects of Examples 1 to 3 and Comparative Examples 1 to 2, and the vertical axis represents the percentage efficiency of current conversion into specific products. The combination of filled square scatter dots and solid lines represents the urea Faraday efficiency, and the combination of filled circular scatter dots and dashed lines represents the hydrogen evolution Faraday efficiency.

[0089] Based on the data in Table 2 and Figure 2It can be seen that different cathode structure designs play a fundamental supporting role in the selection and regulation of reaction pathways. An examination of the test records of Comparative Example 1 revealed that, under conventional aeration methods, the introduced carbon dioxide gas hydrates into bicarbonate ions upon contact with the weakly alkaline wastewater.

[0090] Due to mass transfer losses caused by hydration, free carbon-containing precursors struggle to reach the catalytic activation zone on the electrode surface. Most of the electrons input to the system are captured by interfacial water molecules, and the test system is dominated by a simple hydrogen evolution side reaction. The introduction of a gas diffusion electrode architecture alters the energy distribution pattern of the reaction system. Examples 1 to 3, through the combination of independent gas chambers and porous substrates, opened dedicated transport channels for non-hydrated gaseous molecules, bypassing bulk absorption, and the charge conversion efficiency of urea recovered to the range meeting the preset conditions. In the daily commissioning of electrochemical fluid devices, the problem of water permeation into the electrode pores is frequently encountered. Data from Comparative Example 2 confirms the disruptive effect of water masking on carbon-nitrogen coupling.

[0091] Freed from the surface tension constraint of the hydrophobic polymer, the thickening of the liquid film inside the electrode causes the proton reduction barrier to shift downward, and the active sites are occupied by the associated hydrogen gas. Examples 1 to 3, by controlling the doping ratio of polytetrafluoroethylene, utilize the hydrophobic repulsion force of the polymer chain segments to block the excessive wetting of liquid water, constrain the competition for hydrogen evolution, and anchor the electrical energy that should have been consumed in the side reaction to the conversion process of nitrate and carbon dioxide.

[0092] Furthermore, due to the high complexity of the anaerobic digester substrate, the remaining charge transfer in the system is mainly diverted to the incomplete reduction pathway of nitrate (e.g., generating environmentally harmless nitrogen gas) and carbon-based side reactions such as the conversion of carbon dioxide into trace amounts of carbon monoxide or formate. Despite the aforementioned complex substrate competition and side reaction diversion, thanks to the specific electrode architecture and operational control of this invention, the Faraday efficiency of the target product urea has been successfully anchored and maintained within a highly efficient conversion range of approximately 20%, achieving the technical expectation of nitrogen removal and carbon fixation in complex water bodies.

[0093] Test Example 3:

[0094] The continuous flow electrochemical reaction systems of Examples 1, 2, 3 and Comparative Example 3 were selected as the test objects for the anti-fouling life assessment.

[0095] Aerobic nitrification wastewater containing complex humic macromolecules was continuously pumped into the liquid chamber of the pollution resistance life assessment test object.

[0096] Turn on the DC power supply equipment and maintain the set cathode reduction current density for 120 hours of continuous operation. The solution discharged from the liquid chamber is extracted at fixed time intervals by the automatic sampling valve.

[0097] The concentration of urea in the extracted solution was detected by spectrophotometry, and the urea generation rate at different operating time points was calculated. The ratio of the generation rate measured at each time point to the highest generation rate at the beginning of system startup was obtained to obtain the yield retention rate, which characterizes the catalytic anti-poisoning performance.

[0098] Table 3. Measured data on urea yield retention rate during continuous operation of the example and comparative systems.

[0099] Runtime point (h) Example 1 Yield retention rate (%) Example 2 Yield retention rate (%) Example 3 Yield retention rate (%) Comparative Example 3: Yield retention rate (%) 10 98.4 97.2 99.1 83.6 30 99.2 95.8 96.5 44.2 50 96.7 92.4 94.8 15.1 80 94.5 90.7 91.2 3.8 120 93.1 88.5 89.4 0.4

[0100] The test results are as follows:

[0101] Figure 3 This is a scatter plot of the yield retention rate of the embodiments and comparative examples of the present invention as a function of operating time. The horizontal axis of the figure corresponds to the continuous operation time process, and the vertical axis represents the urea yield retention rate of each test object at a specific time. The broken lines with different combinations of dots and lines depict the evolution trajectory of the anti-pollution lifespan of Example 1, Example 2, Example 3 and Comparative Example 3, respectively.

[0102] Based on the data in Table 3 and Figure 3 It is evident that complex wastewater systems contain high concentrations of humic acid and lipid derivatives, making long-term operational testing a crucial threshold for verifying the practical value of catalytic architecture engineering. Comparative Example 3, without the introduction of a pulse mechanism, experienced chain stagnation after less than 30 hours of operation, exhibiting an irreversible decline in catalytic activity. Disassembly of the failed reactor revealed a dense, dark brown organic colloidal layer adhering to the cathode surface.

[0103] Complex macromolecular organic compounds stubbornly adhere to the periphery of metal active sites through coordination bonds and hydrophobic interactions, constructing an insulating barrier that hinders electron transfer and material diffusion. Exploring ways to break the limitations of conventional continuous reduction modes has become an essential path to extending electrolysis life. The yield retention curves of Examples 1 to 3 depict drastically different evolutionary trends, maintaining above 85% of the initial state even after more than 100 hours of fluid flushing.

[0104] The periodic intervention of a positive potential instantaneously alters the charge distribution at the solid-liquid interface. Brief anodic polarization induces slight oxidative remodeling of the surface metal, causing the macromolecular network attached to the metal surface to detach and enter the bulk fluid due to loss of adhesion. Intermittent electric field reversals clear impurities occupying reaction channels, and the re-exposed catalytic core continues to participate in the carbon dioxide and nitrate conversion cycle at subsequent reduction potentials, establishing a long-term operational mechanism for real-world wastewater systems.

[0105] Test Example 4:

[0106] The continuous flow electrochemical reaction systems of Examples 1, 2, and 3, which operate under long-term constant cathode reduction and superimposed pulse desorption stages, were selected as test objects for evaluating the structural stability and anti-leakage of the metal active components.

[0107] During the operation of the electrochemical reaction system according to the preset operating parameters, the liquid phase water mixture after the gas-liquid-solid three-phase interface reaction is collected at specific time intervals using the drain pipe arranged at the tail end of the electrochemical reactor.

[0108] Nitric acid was added to the collected liquid samples at different time points for digestion pretreatment to destroy the organic complexes that encapsulate metal ions in the water. The total dissolved copper ion concentration in the liquid samples was determined by inductively coupled plasma mass spectrometry.

[0109] The concentration measurements of each embodiment system during continuous operation were summarized and recorded. Combined with the heavy metal loss characteristics of conventional water treatment processes, the structural evolution of the catalyst layer under the impact of a positive pulse electric field was analyzed.

[0110] Table 4. Measured data of total copper concentration in liquid phase effluent during continuous operation of the examples.

[0111] Runtime point (h) Example 1: Total copper concentration in effluent (mg / L) Example 2: Total copper concentration in effluent (mg / L) Example 3: Total copper concentration in effluent (mg / L) 10 0.12 0.21 0.05 24 0.15 0.28 0.08 48 0.11 0.24 0.06 72 0.16 0.31 0.09 96 0.14 0.27 0.07 120 0.13 0.29 0.08

[0112] The test results are as follows:

[0113] Figure 4 This is a graph showing the evolution of total copper concentration in the liquid phase effluent of an embodiment of the present invention. The horizontal axis of the graph represents the time span of continuous operation of the electrochemical reaction system, and the vertical axis corresponds to the total copper ion mass concentration detected in the liquid phase effluent sample. The legend shows three trajectories composed of different scatter points and lines, which respectively record the copper component loss fluctuation state of Examples 1, 2 and 3 during the test period.

[0114] Based on the data in Table 4 and Figure 4 It is known that metal substrates typically face the risk of structural loss due to anodic dissolution corrosion when subjected to positive high-potential impacts. Measuring the migration abundance of transition metal ions in the fluid medium becomes a direct criterion for assessing the reversibility of in-situ interfacial reconstruction reactions. During routine operation and maintenance of industrial water treatment fluid systems containing halide ions, the disordered peeling of the catalyst layer is often observed, leading to a darkening of the fluid color and consequently reducing overall conversion efficiency.

[0115] In Examples 1 to 3, the total copper concentration in the liquid effluent remained below the safety threshold of 0.35 mg / L during a continuous monitoring period of over 100 hours. The measured numerical curves showed slight oscillations and did not exhibit a unidirectional uncontrolled dissipation characteristic that increased over time. The brief positive pulse potential applied to the system did indeed induce a phase transition and reconstruction process for copper oxide formation on the surface of the metal catalytic core. The coordination state of the surface atoms changed, and the phase transition was strictly confined to the shallow surface region, without diffusing into the depth of the electrode framework. After the system switched back to the cathodic reduction electric field environment, the deposited thin layer of divalent and monovalent copper oxides encountered strong electron injection and were restored in situ to an activated zero-valent copper structure containing dense lattice defect states.

[0116] This millisecond-level controlled redox cycle suppressed the irreversible transfer and diffusion of ionic groups into the aqueous bulk. Example 2 employed a relatively high upper limit parameter for the pulse potential, resulting in a high concentration of detached components captured in the liquid phase, while the overall dissolution loss remained within the safe envelope allowed by the structural stability. The technical concept of coupling decontamination and catalytic center regeneration into the same electrical control process fulfills the process requirement of total metal conservation.

Claims

1. A method for the electrocatalytic synthesis of urea from nitrates and carbon dioxide in kitchen wastewater, characterized in that, Includes the following steps: The kitchen waste biogas slurry is subjected to aerobic nitrification and pretreatment to obtain aerobic nitrified biogas slurry wastewater; An electrochemical reactor containing an anolyte chamber, a catholyte chamber, and an independent cathode gas chamber is assembled by combining a gas diffusion electrode cathode containing a copper catalyst, a cation exchange membrane, and a titanium-based ruthenium-iridium anode. The aerobic nitrification biogas slurry wastewater and electrolyte solution are pumped into the cathode liquid chamber and the anolyte liquid chamber respectively, and the pH value of the cathode liquid phase is adjusted to obtain the liquid phase system to be electrolyzed; Carbon dioxide gas is introduced into the independent cathode chamber, and the liquid phase system to be electrolyzed is subjected to cathode reduction operation to produce a urea-containing solution and obtain an electrode system that has been slightly poisoned. A positive pulse potential is applied to the slightly poisoned electrode system for in-situ cleaning and desorption, causing the copper catalyst on the cathode surface to form a copper oxide layer, thus obtaining an electrode system that has completed in-situ cleaning and desorption. The electrode system that has completed in-situ cleaning and desorption is restored to the cathode reduction state, so that the copper oxide layer is reduced to activated zero-valent copper to carry out a coupling reaction and exhaust gas. The discharged catholy liquid is collected to obtain the target urea solution product, and the unreacted carbon dioxide gas in the discharged tail gas is recycled.

2. The method according to claim 1, characterized in that, Prior to the step of assembling the gas diffusion electrode cathode containing the copper catalyst, the cation exchange membrane, and the titanium-based ruthenium-iridium anode, the step of preparing the gas diffusion electrode cathode is included: Copper catalyst powder is mixed with polytetrafluoroethylene hydrophobic binder, and the dry weight mass fraction of polytetrafluoroethylene in the mixture is controlled to be 5% to 15%. The mixture is coated on a copper foam substrate and pressed to obtain the gas diffusion electrode cathode containing the copper catalyst.

3. The method according to claim 1, characterized in that, The pretreatment is a decalcification and magnesium removal pretreatment.

4. The method according to claim 1, characterized in that, The pH value of the cathode liquid phase is adjusted to 8.0 to 10.

0.

5. The method according to claim 1, characterized in that, The carbon dioxide gas is pure carbon dioxide gas; In the step of introducing carbon dioxide gas into the independent cathode gas chamber, the inlet pressure is adjusted to be higher than the static pressure of the liquid column in the cathode liquid chamber of the electrochemical reactor, and the pressure difference is controlled between 2 kPa and 10 kPa.

6. The method according to claim 1, characterized in that, In the step of performing cathodic reduction operation on the liquid phase system to be electrolyzed: The pH value of the cathode liquid phase is kept constant by adding acid-base regulators online, and the salinity balance of the system is maintained by continuous liquid phase feed and discharge.

7. The method according to claim 1, characterized in that, In the step of performing cathodic reduction operation on the liquid phase system to be electrolyzed: The control operating current density is 10 mA / cm². 2 Up to 50mA / cm 2 Perform constant cathode reduction operation for 30 to 60 minutes.

8. The method according to claim 1, characterized in that, In the step of applying a positive pulse potential to the slightly poisoned electrode system for in-situ cleaning and desorption: In-situ cleaning and desorption are performed by applying a positive pulse potential relative to the reversible hydrogen electrode for a duration of 1 to 5 seconds and a potential of +0.5V to +1.0V.

9. The method according to claim 1, characterized in that, In the step of restoring the electrode system to a cathode reduction state after in-situ cleaning and desorption: Restored to a control operating current density of 10 mA / cm² 2 Up to 50mA / cm 2 The constant cathode reduction state.

10. The method according to claim 1, characterized in that, In the step of recycling the unreacted carbon dioxide gas in the exhaust gas: The exhaust gas is monitored in real time using an online gas analyzer, and the unreacted carbon dioxide gas is recycled after gas-liquid separation.

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