Preparation method of urea

Abstract

The invention provides a preparation method of urea, which comprises the following steps: forming an electrode pair with a first counter electrode by taking a solution containing carbon dioxide and nitrogen oxide salt as a first electrolyte and a first working electrode as a cathode, a working voltage of-0.7 V to-1.0 V is applied to the first working electrode, so that carbon dioxide and the nitric oxide-containing salt in the electrolyte are subjected to a co-reduction reaction to obtain urea, and byproducts of carbon monoxide and ammonia are generated; a reaction solution after the co-reduction reaction is completed is used as a second electrolyte, a second working electrode is used as an anode to form an electrode pair with a second counter electrode, a working voltage of 0.6-0.9 V is applied to the second working electrode, so that carbon monoxide and ammonia in the second electrolyte are subjected to a co-oxidation reaction to obtain urea, and by-products carbon dioxide and nitrogen-containing oxide salt are generated; and taking the reaction liquid after the co-oxidation reaction as a first electrolyte, and alternately and repeatedly carrying out the co-reduction reaction and the co-oxidation reaction. The conversion rate of the urea prepared by the preparation method is high.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of urea synthesis, in particular to a preparation method of urea. BACKGROUND

[0002] Nitrogen fertilizer is an essential fertilizer for agricultural production, and urea is one of the most important nitrogen fertilizers due to its high nitrogen content. Therefore, the development of urea industry is of great significance to meet the growing population demand. At present, humans mainly prepare urea through the reaction of ammonia (NH3) and carbon dioxide (CO2) under high temperature (150℃-200℃) and high pressure (150bar-250bar) reaction conditions. This method not only has high energy consumption, but also causes low reaction efficiency due to complex reaction equipment and synthesis process. Therefore, it will be a research hotspot in this field in the future to prepare urea in a more moderate, energy-saving and environmentally friendly way.

[0003] In recent years, researchers have made many key progress in the study of electrocatalytic carbon-nitrogen coupling to prepare urea. In this field, carbon sources (such as carbon dioxide and carbon monoxide (CO)) and nitrogen sources (such as nitrogen (N2), nitrate (NO x ) are fully explored for electrocatalytic coupling, and most of the work can prepare urea product in the form of green electrocatalytic coupling at room temperature.

[0004] The electrocatalytic carbon-nitrogen coupling technology for synthesizing urea uses cleaner electric energy as the driving force, which is in line with the future trend of green and environmentally friendly energy development. However, the coupling process of carbon dioxide and nitrogen / nitrate involves multi-electron transfer, and the reaction process is complex, which has the problem of low conversion rate, and there is still a lot of room for improvement before practical application. SUMMARY

[0005] Therefore, the present application provides a preparation method of urea, which has a high conversion rate of urea.

[0006] The technical scheme provided by the present application is as follows:

[0007] According to the first aspect of the present application, a preparation method of urea is provided, which comprises the following steps:

[0008] The solution containing carbon dioxide and nitrogen oxide salt is used as the first electrolyte, the first working electrode is used as the cathode to form an electrode pair with the first counter electrode, and a working voltage of-0.7V to-1.0V is applied to the first working electrode to make the carbon dioxide and nitrogen oxide salt in the first electrolyte undergo a co-reduction reaction to obtain urea, and by-products carbon monoxide and ammonia are generated at the same time;

[0009] The reaction solution after the completion of the co-reduction reaction is used as a second electrolyte, a second working electrode is used as an anode to form an electrode pair with a second counter electrode, and a working voltage of 0.6V-0.9V is applied to the second working electrode to make carbon monoxide and ammonia in the second electrolyte undergo a co-oxidation reaction to obtain urea, and simultaneously generate by-products carbon dioxide and nitrogen oxide salt;

[0010] The reaction solution after the completion of the co-oxidation reaction is used as the first electrolyte, and the co-reduction reaction and the co-oxidation reaction are alternately repeated.

[0011] In any embodiment, the solution containing carbon dioxide and nitrogen oxide salt is a carbon dioxide-saturated aqueous nitrate and / or nitrite solution.

[0012] In any embodiment, the working voltage applied to the first working electrode is-0.9V- -1.0V; and / or, the working voltage applied to the second working electrode is 0.7V-0.8V.

[0013] In any embodiment, the first working electrode comprises a titanium dioxide electrode in anatase form, and the second working electrode comprises a platinum-carbon electrode.

[0014] In any embodiment, the co-reduction reaction and the co-oxidation reaction are carried out in the same pendulum electrolysis device, the pendulum electrolysis device comprises a first electrolytic cell, a flow field plate, and a second electrolytic cell, the flow field plate is arranged between the first electrolytic cell and the second electrolytic cell, and the flow field plate has a flow channel for the flow of electrolyte;

[0015] The first electrolytic cell comprises a first electrolysis chamber, and the first working electrode, the first counter electrode, and a first ion exchange membrane arranged in the first electrolysis chamber, the first ion exchange membrane divides the inner cavity of the first electrolysis chamber into a first chamber and a second chamber, the second chamber is in communication with the flow channel, the first working electrode is arranged in the second chamber, and the first counter electrode is arranged in the first chamber.

[0016] The second electrolytic cell comprises a second electrolysis chamber, and the second working electrode, the second counter electrode, and a second ion exchange membrane arranged in the second electrolysis chamber, the second ion exchange membrane divides the inner cavity of the second electrolysis chamber into a third chamber and a fourth chamber, the third chamber is in communication with the flow channel, the second working electrode is arranged in the third chamber, and the second counter electrode is arranged in the fourth chamber.

[0017] In any embodiment, the flow channel is arranged in a serpentine shape on the flow field plate; in the thickness direction of the flow field plate, the flow channel penetrates through the flow field plate and is in communication with the second chamber and the third chamber.

[0018] In any of the embodiments, the first electrolytic cell further comprises a first end plate and a first supporting electrolyte cavity plate, the first supporting electrolyte cavity plate is provided with a first supporting electrolyte containing cavity penetrating through the two side walls of the first supporting electrolyte cavity plate, the first end plate, the first supporting electrolyte cavity plate and the flow field plate are sequentially stacked and connected to form the first electrolytic chamber; the first ion exchange membrane is arranged between the first supporting electrolyte cavity plate and the flow field plate, the first working electrode is arranged between the first ion exchange membrane and the flow field plate, and the first counter electrode is arranged in the first supporting electrolyte containing cavity.

[0019] In any of the embodiments, the first electrolytic cell further comprises a first working electrode catalytic layer and a first counter electrode catalytic layer, the first working electrode catalytic layer is arranged between the flow field plate and the first ion exchange membrane, and is used for catalyzing the electrode reaction on the first working electrode; the first end plate is provided with a first catalyst containing cavity on the side close to the first supporting electrolyte cavity plate, and the first counter electrode catalytic layer is arranged in the first catalyst containing cavity, and the first counter electrode catalytic layer is used for catalyzing the electrode reaction on the first counter electrode.

[0020] In any of the embodiments, the second electrolytic cell further comprises a second end plate and a second supporting electrolyte cavity plate, the second supporting electrolyte cavity plate is provided with a second supporting electrolyte containing cavity penetrating through the two side walls of the second supporting electrolyte cavity plate, the second end plate, the second supporting electrolyte cavity plate and the flow field plate are sequentially stacked and connected to form the second electrolytic chamber; the second ion exchange membrane is arranged between the second supporting electrolyte cavity plate and the flow field plate, the second working electrode is arranged between the second ion exchange membrane and the flow field plate, and the second counter electrode is arranged in the second supporting electrolyte containing cavity.

[0021] In any of the embodiments, the second electrolytic cell further comprises a second working electrode catalytic layer and a second counter electrode catalytic layer, the second working electrode catalytic layer is arranged between the flow field plate and the second ion exchange membrane, and is used for catalyzing the electrode reaction on the second working electrode; the second end plate is provided with a second catalyst containing cavity on the side close to the second supporting electrolyte cavity plate, and the second counter electrode catalytic layer is arranged in the second catalyst containing cavity, and the second counter electrode catalytic layer is used for catalyzing the electrode reaction on the second counter electrode.

[0022] Compared with the conventional technology, the present application has at least the following beneficial effects:

[0023] The first electrolyte containing carbon dioxide and nitrogen oxide salt is used, and the co-reduction reaction is carried out at the cathode under the working voltage of-0.7V-1.0V to obtain urea, and a reaction liquid containing by-products carbon monoxide and ammonia is obtained; the reaction liquid is used as the second electrolyte, and the co-oxidation reaction is carried out at the anode under the working voltage of 0.6V-0.9V to obtain urea, and a reaction liquid containing by-products carbon dioxide and nitrogen oxide salt is obtained; the reaction liquid after the completion of the co-oxidation reaction is used as the first electrolyte to alternately repeat the above-mentioned co-reduction reaction and co-oxidation reaction; in this way, the co-reduction reaction and the co-oxidation reaction are repeatedly carried out in a pendulum mode, the by-products (carbon monoxide and ammonia) of the co-reduction reaction are used as the reaction raw materials of the co-oxidation reaction, and the by-products (carbon dioxide and nitrogen oxide salt) of the co-oxidation reaction are used as the reaction raw materials of the co-reduction reaction; the conversion rate of preparing urea from the nitrogen oxide salt can be greatly improved. BRIEF DESCRIPTION OF DRAWINGS

[0024] In order to better describe and illustrate the embodiments or examples provided in the present application, one or more drawings can be referred to. Additional details or examples used to describe the drawings should not be considered as limiting the scope of any one of the disclosed applications, the presently described embodiments or examples, and the best mode presently understood of these applications. Moreover, in all the drawings, the same reference numbers are used to represent the same components. In the drawings:

[0025] Figure 1 It is a schematic diagram of the pendulum electrolysis device of an embodiment of the present application;

[0026] Figure 2 It is a schematic diagram of the first end plate in the pendulum electrolysis device of an embodiment of the present application;

[0027] Figure 3 It is a schematic diagram of the first support electrolyte cavity plate in the pendulum electrolysis device of an embodiment of the present application;

[0028] Figure 4 It is a schematic diagram of the flow field plate in the pendulum electrolysis device of an embodiment of the present application;

[0029] Figure 5 It is a SEM diagram of the titanium dioxide electrode used in the preparation method of an embodiment of the present application;

[0030] Figure 6 It is an XRD diagram of the titanium dioxide electrode used in the preparation method of an embodiment of the present application;

[0031] Figure 7 It is a comparison diagram of the generation rate and Faraday efficiency of the titanium dioxide electrode in Examples 1-4 of the present application under different working voltages to prepare urea;

[0032] Figure 8 Figure 6 is a plot of the generation rate and Faraday efficiency of urea prepared by the platinum carbon electrode in Example 1~4 of the present application at different working voltages;

[0033] Figure 9 Figure 7 is a plot of the current / time of the pendulum electrolysis device in Example 3 of the present application within 400 seconds of operation;

[0034] Figure 10 Figure 8 is a plot of the UV signal of the urea product obtained over time of operation in Example 3 of the present application.

[0035] BRIEF DESCRIPTION OF DRAWINGS

[0036] 10, pendulum electrolysis device; 11, first electrolytic cell; 12, flow field plate; 13, second electrolytic cell;

[0037] 112, first working electrode; 114, first ion exchange membrane; 115, first counter electrode catalytic layer; 116, first gasket; 1111, first end plate; 1112, first supporting electrolyte cavity plate; 1113, first electrolyte inlet; 1114, first electrolyte outlet; 1111a, first catalyst containing cavity; 1112a, first supporting electrolyte containing cavity;

[0038] 121, flow channel;

[0039] 132, second working electrode; 134, second ion exchange membrane; 135, second counter electrode catalytic layer; 136, second gasket; 1311, second end plate; 1312, second supporting electrolyte cavity plate; 1311a, second catalyst containing cavity; 1312a, second supporting electrolyte containing cavity. DETAILED DESCRIPTION

[0040] In order to make the above objectives, features and advantages of the present application more apparent, the specific embodiments of the present application are described in detail below. In the following description, a lot of specific details are set forth in order to provide a thorough understanding of the present application. However, the present application can be implemented in many different ways than those described herein, and those skilled in the art can make similar improvements without departing from the spirit of the present application, and therefore the present application is not limited to the specific embodiments disclosed below.

[0041] In addition, the terms "first", "second" are only for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined with "first", "second" can explicitly or implicitly include at least one of the features. In the description of the present application, the meaning of "a plurality of" is at least two, for example, two, three, etc., unless otherwise explicitly and specifically limited.

[0042] In this application, unless specifically defined otherwise and limited, the terms "mounting", "connection", "connecting", "fixing" and the like should be interpreted broadly, for example, can be fixed connection, can also be detachable connection, or integrated; can be mechanical connection, can also be electrical connection; can be directly connected, can also be indirectly connected through an intermediate medium, can be the internal communication of two elements or the interaction relationship of two elements, unless otherwise explicitly limited. For those skilled in the art, the specific meaning of the above terms in this application can be understood according to the specific circumstances.

[0043] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the specification of the present application is only for the purpose of describing specific embodiments and is not intended to limit the present application. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0044] In an embodiment of the present application, a method for preparing urea is provided, comprising the following steps:

[0045] The solution containing carbon dioxide and nitrogen-containing oxide salt is used as the first electrolyte, the first working electrode is used as the cathode to form an electrode pair with the first counter electrode, and a working voltage of-0.7 V to-1.0 V is applied to the first working electrode to make the carbon dioxide and nitrogen-containing oxide salt in the first electrolyte undergo a co-reduction reaction to obtain urea, and by-products carbon monoxide and ammonia are generated at the same time;

[0046] The reaction solution after the completion of the co-reduction reaction is used as the second electrolyte, the second working electrode is used as the anode to form an electrode pair with the second counter electrode, and a working voltage of 0.6 V to 0.9 V is applied to the second working electrode to make the carbon monoxide and ammonia in the second electrolyte undergo a co-oxidation reaction to obtain urea, and by-products carbon dioxide and nitrogen-containing oxide salt are generated at the same time;

[0047] The reaction solution after the completion of the co-oxidation reaction is used as the first electrolyte, and the co-reduction reaction and the co-oxidation reaction are alternately repeated.

[0048] The above-mentioned method for preparing urea uses the first electrolyte containing carbon dioxide and nitrogen-containing oxide salt, electrolysis is carried out at the cathode under a working voltage of-0.7 V to-1.0 V to make the carbon dioxide and nitrogen-containing oxide salt undergo a co-reduction reaction to obtain urea, and a reaction solution containing by-products carbon monoxide and ammonia is obtained at the same time; the reaction solution is used as the second electrolyte, electrolysis is carried out at the anode under a working voltage of 0.6 V to 0.9 V to make the carbon monoxide and ammonia undergo a co-oxidation reaction to obtain urea, and a reaction solution containing by-products carbon dioxide and nitrogen-containing oxide salt is obtained at the same time; the reaction solution after the completion of the co-oxidation reaction is used as the first electrolyte, and the above-mentioned co-reduction reaction and co-oxidation reaction are alternately repeated; the reaction is ended after reaching a predetermined conversion rate.

[0049] In this way, the co-reduction reaction and the co-oxidation reaction are repeatedly performed in a pendulum manner, the by-products (carbon monoxide and ammonia) of the co-reduction reaction are used as the reaction raw materials of the co-oxidation reaction, and the by-products (carbon dioxide and nitrogen oxide salt) of the co-oxidation reaction are used as the reaction raw materials of the co-reduction reaction, so that the conversion rate of the nitrogen oxide salt to urea can be greatly improved.

[0050] Further, the working voltage applied to the first working electrode is preferably -0.9V to -1.0V. Controlling the working voltage of the first working electrode within the above range is conducive to making the co-reduction reaction for preparing urea have a higher generation rate and Faraday efficiency.

[0051] Further, the working voltage applied to the second working electrode is preferably 0.7V to 0.8V. Controlling the working voltage of the second working electrode within the above range is conducive to making the co-oxidation reaction for preparing urea have a higher generation rate and Faraday efficiency.

[0052] In some embodiments, the solution containing carbon dioxide and nitrogen oxide salt is a carbon dioxide-saturated aqueous nitrate and / or nitrite solution. Specifically, when the co-reduction reaction is performed, carbon dioxide gas can be introduced into the first electrolyte to keep the carbon dioxide in the first electrolyte in a saturated state. In this way, the conversion rate of the nitrogen oxide salt can be further improved. The nitrogen oxide salt can be one or more of a nitrate and a nitrite; the nitrate includes but is not limited to potassium nitrate; the nitrite includes but is not limited to potassium nitrite.

[0053] In some embodiments, the completion of the co-reduction reaction and the co-oxidation reaction can be determined by the following method: when it is detected that the nitrogen oxide salt in the first electrolyte is completely converted, the co-reduction reaction is completed; when it is detected that the ammonia or carbon monoxide in the second electrolyte is completely converted, the co-oxidation reaction is completed. In this way, the conversion rate of the nitrogen oxide salt can be further improved.

[0054] In other embodiments, a reaction time can be preset, and when the co-reduction reaction reaches the preset time, it is considered that the co-reduction reaction is completed; when the co-oxidation reaction reaches the preset time, it is considered that the co-oxidation reaction is completed. The preset time of the co-reduction reaction and the preset time of the co-oxidation reaction can be the same or different. In some specific examples, the preset time of the co-reduction reaction and the preset time of the co-oxidation reaction are both 100s.

[0055] In some embodiments, the first working electrode comprises a titanium dioxide electrode in anatase form, and the second working electrode comprises a platinum-carbon electrode. The titanium dioxide electrode in anatase form has good electrochemical activity for the co-reduction reaction of carbon dioxide and the nitrate salt to prepare urea at a working voltage of -0.7 V to -1.0 V. The platinum-carbon electrode has good electrochemical activity for the co-oxidation reaction of carbon monoxide and ammonia to prepare urea at a working voltage of 0.6 V to 0.9 V.

[0056] In some embodiments, the first counter electrode and the second counter electrode both comprise a graphite electrode.

[0057] Please refer to Figure 1 , Figure 2 , Figure 3 and Figure 4 In some embodiments, the co-reduction reaction and the co-oxidation reaction are carried out in the same pendulum electrolysis device 10, which comprises a first electrolytic cell 11, a flow field plate 12, and a second electrolytic cell 13. The flow field plate 12 is arranged between the first electrolytic cell 11 and the second electrolytic cell 13, and has flow channels 121 for the electrolyte to flow. The first electrolytic cell 11 comprises a first electrolytic chamber (not shown in the figure), a first working electrode 112, a first counter electrode (not shown in the figure), and a first ion exchange membrane 114 arranged in the first electrolytic chamber. The first ion exchange membrane 114 divides the inner cavity of the first electrolytic chamber into a first chamber (not shown in the figure) and a second chamber (not shown in the figure). The second chamber is in communication with the flow channels 121. The first working electrode 112 is arranged in the second chamber, and the first counter electrode is arranged in the first chamber. The second electrolytic cell 13 comprises a second electrolytic chamber (not shown in the figure), a second working electrode 132, a second counter electrode, and a second ion exchange membrane 134 arranged in the second electrolytic chamber. The second ion exchange membrane 134 divides the inner cavity of the second electrolytic chamber into a third chamber and a fourth chamber. The third chamber is in communication with the flow channels 121. The second working electrode 132 is arranged in the third chamber, and the second counter electrode is arranged in the fourth chamber.

[0058] By carrying out the co-reduction reaction and the co-oxidation reaction in the same pendulum electrolysis device 10 as described above, the co-reduction reaction of carbon dioxide and the nitrate salt to prepare urea is carried out in the first electrolytic cell 11, and the co-oxidation reaction of carbon monoxide and ammonia to prepare urea is carried out in the second electrolytic cell 13. The first electrolyte is introduced into the flow channels 121 of the flow field plate 12 to pendulumly carry out the co-reduction reaction and the co-oxidation reaction. By pendulumly and alternately repeating the co-reduction reaction and the co-oxidation reaction using the pendulum electrolysis device 10 as described above, the conversion rate of preparing urea from the nitrate salt can be greatly improved.

[0059] In some embodiments, the flow channel 121 extends through the flow field plate 12 along the thickness direction of the flow field plate 12 and communicates with the second chamber and the third chamber. In this way, the flow channel 121 communicates with the second chamber and the third chamber on two sides of the flow field plate 12, respectively, and the electrolyte in the flow channel 121 can enter the second chamber and the third chamber from the sides of the flow channel 121 when flowing in the flow channel 121, and the corresponding electrochemical reduction reaction or electrochemical oxidation reaction can be carried out in the flow channel 121 and the second chamber and the third chamber.

[0060] In some embodiments, the flow channel 121 is arranged in a serpentine shape on the flow field plate 12. In this way, the reactants in the electrolyte in the flow channel 121 can more fully participate in the electrode reaction, thereby facilitating further improvement of the conversion rate of the electrochemical reaction.

[0061] In some embodiments, the first electrolytic cell 11 further comprises a first end plate 1111 and a first supporting electrolyte chamber plate 1112, the first supporting electrolyte chamber plate 1112 is provided with a first supporting electrolyte containing cavity 1112a extending through the two side walls of the first supporting electrolyte chamber plate 1112, the first end plate 1111, the first supporting electrolyte chamber plate 1112 and the flow field plate 12 are sequentially stacked and connected to form a first electrolysis chamber; the first ion exchange membrane 114 is arranged between the first supporting electrolyte chamber plate 1112 and the flow field plate 12, the first working electrode 112 is arranged between the first ion exchange membrane 114 and the flow field plate 12, and the first counter electrode is arranged in the first supporting electrolyte containing cavity 1112a.

[0062] In this way, the first end plate 1111, the first supporting electrolyte chamber plate 1112 and the flow field plate 12 are sequentially stacked and connected to form a first electrolysis chamber; the first ion exchange membrane 114 is arranged between the first supporting electrolyte chamber plate 1112 and the flow field plate 12, and the inner cavity of the first electrolysis chamber is divided into the first chamber and the second chamber. The first electrolytic cell 11 has a simple and compact structure. It can be understood that the first end plate 1111, the first supporting electrolyte chamber plate 1112 and the flow field plate 12 can be connected by bolts, and sealing glue can be applied on the abutting sides of the adjacent plates for sealing connection. The first supporting electrolyte containing cavity 1112a is used to hold the supporting electrolyte for maintaining the charge balance in the first electrolysis chamber. The supporting electrolyte can be an aqueous solution containing one or more solutes of potassium bicarbonate, potassium nitrate and potassium nitrite, and the solute concentration can be 0.001 mol / L to 1 mol / L.

[0063] In some embodiments, the first electrolysis cell 11 further comprises a first working electrode catalytic layer (not shown) disposed between the flow field plate 12 and the first ion exchange membrane 114, for catalyzing the electrode reaction on the first working electrode 112. By disposing the first working electrode catalytic layer, the reaction efficiency and conversion rate can be further improved. Specifically, the first working electrode catalytic layer can be made of copper foam.

[0064] In some embodiments, the first electrolysis cell 11 further comprises a first counter electrode catalytic layer 115, the first end plate 1111 is provided with a first catalyst accommodating cavity 1111a near one side of the first supporting electrolyte cavity plate 1112, and the first counter electrode catalytic layer 115 is disposed in the first catalyst accommodating cavity 1111a, for catalyzing the electrode reaction on the first counter electrode. By the first counter electrode catalytic layer 115, the electrochemical reaction can be further promoted. Specifically, the first counter electrode catalytic layer 115 can be a commercial titanium mesh loaded with catalyst.

[0065] In some embodiments, the second electrolysis cell 13 further comprises a second end plate 1311 and a second supporting electrolyte cavity plate 1312, the second supporting electrolyte cavity plate 1312 is provided with a second supporting electrolyte accommodating cavity 1312a penetrating through the two side walls of the second supporting electrolyte cavity plate 1312, the second end plate 1311, the second supporting electrolyte cavity plate 1312 and the flow field plate 12 are sequentially stacked and connected to form a second electrolysis chamber; the second ion exchange membrane 134 is disposed between the second supporting electrolyte cavity plate 1312 and the flow field plate 12, the second working electrode 132 is disposed between the second ion exchange membrane 134 and the flow field plate 12, and the second counter electrode is disposed in the second supporting electrolyte accommodating cavity 1312a.

[0066] In this way, the second electrolysis cell 13 is formed by sequentially stacking and connecting the second end plate 1311, the second supporting electrolyte cavity plate 1312 and the flow field plate 12; the second ion exchange membrane 134 is disposed between the second supporting electrolyte cavity plate 1312 and the flow field plate 12, and the inner cavity of the second electrolysis chamber is divided into a third cavity and a fourth cavity. The second electrolysis cell 13 has a simple and compact structure. It can be understood that the second end plate 1311, the second supporting electrolyte cavity plate 1312 and the flow field plate 12 can be connected by bolts, and sealing glue can be applied on the abutting sides of the adjacent plates for sealing connection. The second supporting electrolyte accommodating cavity is used to hold supporting electrolyte, for maintaining the charge balance in the second electrolysis chamber. The supporting electrolyte can be an aqueous solution containing one or more solutes of potassium bicarbonate, potassium nitrate and potassium nitrite, and the solute concentration can be 0.001 mol / L to 1 mol / L.

[0067] In some embodiments, the second electrolysis cell 13 further comprises a second working electrode catalytic layer (not shown in the figure) arranged between the flow field plate 12 and the second ion exchange membrane 134, for catalyzing the electrode reaction on the second working electrode 132. By arranging the second working electrode catalytic layer, the reaction efficiency and conversion rate can be further improved. Specifically, the second working electrode catalytic layer can be made of nickel foam.

[0068] In some embodiments, the second electrolysis cell 13 further comprises a second counter electrode catalytic layer 135, the second end plate 1311 is provided with a second catalyst accommodating cavity 1311a near one side of the second supporting electrolyte cavity plate 1312, and the second counter electrode catalytic layer 135 is arranged in the second catalyst accommodating cavity 1311a, and the second counter electrode catalytic layer 135 is used to catalyze the electrode reaction on the second counter electrode 133. By arranging the second counter electrode catalytic layer 135, the electrochemical reaction can be further promoted. Specifically, the second counter electrode catalytic layer 135 can be a commercial titanium mesh loaded with catalyst.

[0069] In some embodiments, the first end plate 1111, the first supporting electrolyte cavity plate 1112 and the flow field plate 12 are respectively provided with a first electrolyte inlet 1113 and a first electrolyte outlet 1114 corresponding in position, and the first electrolyte inlet 1113 and the first electrolyte outlet 1114 are respectively connected to both ends of the flow channel 121.

[0070] In this way, electrolyte can be introduced into the flow channel 121 through the first electrolyte inlet 1113, and electrochemical reaction can be carried out in the first electrolysis cell 11 or the second electrolysis cell 13, and the electrolyte in the flow channel 121 can be discharged through the first electrolyte outlet 1114. The electrolyte can be subjected to electrochemical reaction in a flowing state. It can be understood that a valve can be arranged to control the entry of electrolyte into the flow channel 121 through the first electrolyte inlet 1113, and a valve can be arranged to control the discharge of electrolyte through the first electrolyte outlet 1114.

[0071] In some embodiments, a first gasket 116 with a through inner cavity in the middle can be arranged between the first end plate 1111 and the first supporting electrolyte cavity plate 1112, between the first supporting electrolyte cavity plate 1112 and the first working electrode 112, and between the first working electrode 112 and the flow field plate 12. Similarly, a second gasket 136 with a through inner cavity in the middle can be arranged between the second end plate 1311 and the second supporting electrolyte cavity plate 1312, between the second supporting electrolyte cavity plate 1312 and the second working electrode 132, and between the second working electrode 132 and the flow field plate 12.

[0072] The through inner cavity of the first gasket 116 can be used as a part of the first electrolysis chamber, and the through inner cavity of the second gasket 136 can be used as a part of the second electrolysis chamber. The first gasket 116 is also provided with a first electrolyte inlet 1113 corresponding to the position of the first electrolyte inlet 1113 on the first end plate 1111, the first support electrolyte cavity plate 1112 and the flow field plate 12, and a first electrolyte outlet 1114 corresponding to the position of the first electrolyte outlet 1114 in the above structure.

[0073] The application will be further described in conjunction with specific examples below, but should not be construed as limiting the scope of protection of the application.

[0074] Example 1

[0075] Step one:

[0076] The pendulum type electrolysis device is used for preparing urea. The carbon dioxide saturated potassium nitrate aqueous solution is used as the first electrolyte. The first electrolyte is introduced into the flow channel of the flow field plate from the first electrolyte inlet on the first end plate. The first pair of electrodes is graphite electrodes, and the support electrolyte at the first pair of electrodes is a 0.05 mol / L potassium nitrate aqueous solution. A working voltage of-0.7 V is applied to the first working electrode (anatase titanium dioxide electrode) by the power supply to carry out the co-reduction reaction of carbon dioxide and nitrate to obtain the product urea. At the same time, carbon monoxide and ammonia are generated and dissolved in the electrolyte, and the reaction time is 100 s.

[0077] Step two:

[0078] After the completion of the co-reduction reaction in step one, the power supply to the first working electrode is stopped. The reaction solution after the completion of the co-reduction reaction is used as the second electrolyte. The second pair of electrodes is graphite electrodes, and the support electrolyte at the second pair of electrodes is a 0.05 mol / L potassium nitrate aqueous solution. A working voltage of 0.6 V is applied to the second working electrode (platinum-carbon electrode) by the power supply to carry out the co-oxidation reaction of carbon monoxide and ammonia to obtain the product urea. At the same time, carbon dioxide and nitrogen-containing oxide salt (such as nitrate, nitrite) are generated and dissolved in the electrolyte, and the reaction time is 100 s.

[0079] Step three:

[0080] The reaction solution after the completion of the co-oxidation reaction in step two is used as the first electrolyte, and the co-reduction reaction is carried out according to the method of step one for 100 s.

[0081] Step four:

[0082] The reaction solution after the completion of the co-reduction reaction in step three is used as the second electrolyte, and the co-oxidation reaction is carried out according to the method of step two for 100 s.

[0083] After the reaction, the total conversion rate of potassium nitrate to urea is close to 100%.

[0084] Example 2

[0085] This example is basically the same as Example 1, except that in Step 1 and Step 3, the working voltage applied to the first working electrode is -0.8 V; in Step 2 and Step 4, the working voltage applied to the second working electrode is 0.7 V.

[0086] After the reaction, the total conversion rate of potassium nitrate to urea is close to 100%.

[0087] Example 3

[0088] This example is basically the same as Example 1, except that in Step 1 and Step 3, the working voltage applied to the first working electrode is -0.9 V; in Step 2 and Step 4, the working voltage applied to the second working electrode is 0.8 V.

[0089] After the reaction, the total conversion rate of potassium nitrate to urea is close to 100%.

[0090] Example 4

[0091] This example is basically the same as Example 1, except that in Step 1 and Step 3, the working voltage applied to the first working electrode is -1.0 V; in Step 2 and Step 4, the working voltage applied to the second working electrode is 0.9 V.

[0092] After the reaction, the total conversion rate of potassium nitrate to urea is close to 100%.

[0093] Figure 5 and Figure 6 respectively are the SEM and XRD images of the titanium dioxide electrode used as the first working electrode in this application; from Figure 5 It can be seen that the titanium dioxide electrode has a nanosheet morphology; from Figure 6 It can be seen that the titanium dioxide therein is of anatase crystal form.

[0094] Figure 7 is a comparison chart of the electrochemical activity of the titanium dioxide electrode for preparing urea under different working voltages in Examples 1-4 of this application. From Figure 7 It can be seen that the titanium dioxide electrode has good electrochemical activity for preparing urea in the voltage range of -0.7 V to -1.0 V, and has a higher generation rate and Faraday efficiency in the voltage range of -0.9 V to -1.0 V.

[0095] Figure 8 is a comparison chart of the electrochemical activity of the platinum-carbon electrode (Pt / C) for preparing urea under different working voltages in Examples 1-4 of this application. FromFigure 8 It can be seen that the platinum carbon electrode has good electrochemical activity for preparing urea in the voltage range of 0.6V-0.9V, and has a higher generation rate and Faraday efficiency in the voltage range of 0.7V-0.8V.

[0096] Figure 9 is a current / time schematic diagram of the pendulum reaction 400s in Embodiment 3 of the present application. From Figure 9 It can be seen that the pendulum electrolysis device can be stably operated at the oxidation / reduction potential. Figure 10 is a UV signal diagram of the urea product obtained by running with the reaction time in Embodiment 3 of the present application. From Figure 10 It can be seen that the urea product obtained at the oxidation / reduction potential gradually increases with the running time.

[0097] The technical features of the above-described embodiments can be combined arbitrarily. In order to make the description simple, all possible combinations of the technical features in the above-described embodiments are not described, however, as long as the combinations of the technical features do not contradict, they should be considered as the scope of the present application.

[0098] The above-described embodiments only express several implementation manners of the present application, and the description is more specific and detailed, but it should not be understood as a limitation on the scope of the patent of the present application. It should be pointed out that for ordinary skilled in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which all belong to the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the appended claims, and the description and drawings can be used to explain the content of the claims.

Claims

1. A method for preparing urea, characterized in that, Includes the following steps: A solution containing carbon dioxide and nitrogen oxide salts is used as the first electrolyte. A first working electrode is used as the cathode to form an electrode pair with a first pair of electrodes. A working voltage of -0.7V to -1.0V is applied to the first working electrode to cause the carbon dioxide and nitrogen oxide salts in the first electrolyte to undergo a co-reduction reaction to obtain urea, while generating byproducts carbon monoxide and ammonia. The reaction solution after the co-reduction reaction is used as the second electrolyte. The second working electrode is used as the anode to form an electrode pair with the second pair of electrodes. A working voltage of 0.6V to 0.9V is applied to the second working electrode to cause the carbon monoxide and ammonia in the second electrolyte to undergo a co-oxidation reaction to obtain urea, while generating byproducts carbon dioxide and nitrogen-containing oxide salts. Using the reaction solution after the co-oxidation reaction as the first electrolyte, the co-reduction reaction and the co-oxidation reaction are carried out alternately and repeatedly.

2. The method for preparing urea according to claim 1, characterized in that, The solution containing carbon dioxide and nitrogen oxide salts is a carbon dioxide-saturated aqueous solution of nitrates and / or nitrites.

3. The method for preparing urea according to claim 1, characterized in that, The working voltage applied to the first working electrode is -0.9V to -1.0V; and / or, the working voltage applied to the second working electrode is 0.7V to 0.8V.

4. The method for preparing urea according to claim 1, characterized in that, The first working electrode includes anatase titanium dioxide electrode, and the second working electrode includes platinum carbon electrode.

5. The method for preparing urea according to any one of claims 1 to 4, characterized in that, The co-reduction reaction and the co-oxidation reaction are carried out in a pendulum electrolysis device, which includes a first electrolysis cell, a flow field plate and a second electrolysis cell. The flow field plate is disposed between the first electrolysis cell and the second electrolysis cell and has a flow channel for the flow of electrolyte. The first electrolytic cell includes a first electrolytic chamber and a first working electrode, a first pair of electrodes and a first ion exchange membrane disposed in the first electrolytic chamber. The first ion exchange membrane divides the inner cavity of the first electrolytic chamber into a first chamber and a second chamber. The second chamber is connected to the flow channel. The first working electrode is disposed in the second chamber and the first pair of electrodes is disposed in the first chamber. The second electrolytic cell includes a second electrolytic chamber and a second working electrode, a second pair of electrodes, and a second ion exchange membrane disposed within the second electrolytic chamber. The second ion exchange membrane divides the inner cavity of the second electrolytic chamber into a third chamber and a fourth chamber. The third chamber is connected to the flow channel. The second working electrode is disposed within the third chamber, and the second pair of electrodes is disposed within the fourth chamber.

6. The method for preparing urea according to claim 5, characterized in that, The flow channel is arranged in a serpentine bend on the flow field plate; along the thickness direction of the flow field plate, the flow channel penetrates the flow field plate and communicates with the second chamber and the third chamber.

7. The method for preparing urea according to claim 5, characterized in that, The first electrolytic cell further includes a first end plate and a first supporting electrolyte chamber plate. The first supporting electrolyte chamber plate is provided with a first supporting electrolyte receiving cavity penetrating both side walls of the first supporting electrolyte chamber plate. The first end plate, the first supporting electrolyte chamber plate, and the flow field plate are stacked and connected in sequence to form the first electrolytic chamber. The first ion exchange membrane is disposed between the first supporting electrolyte chamber plate and the flow field plate. The first working electrode is disposed between the first ion exchange membrane and the flow field plate. The first pair of electrodes is disposed in the first supporting electrolyte receiving cavity.

8. The method for preparing urea according to claim 7, characterized in that, The first electrolytic cell further includes a first working electrode catalytic layer and a first pair of electrode catalytic layers. The first working electrode catalytic layer is disposed between the flow field plate and the first ion exchange membrane and is used to catalyze the electrode reaction on the first working electrode. A first catalyst receiving cavity is provided on the side of the first end plate near the first supporting electrolyte chamber plate. The first pair of electrode catalytic layers are disposed in the first catalyst receiving cavity and are used to catalyze the electrode reaction on the first pair of electrodes.

9. The method for preparing urea according to claim 5, characterized in that, The second electrolytic cell further includes a second end plate and a second supporting electrolyte chamber plate. The second supporting electrolyte chamber plate is provided with a second supporting electrolyte receiving cavity that penetrates both sides of the second supporting electrolyte chamber plate. The second end plate, the second supporting electrolyte chamber plate, and the flow field plate are stacked and connected in sequence to form the second electrolytic chamber. The second ion exchange membrane is disposed between the second supporting electrolyte chamber plate and the flow field plate. The second working electrode is disposed between the second ion exchange membrane and the flow field plate. The second pair of electrodes is disposed in the second supporting electrolyte receiving cavity.

10. The method for preparing urea according to claim 9, characterized in that, The second electrolytic cell further includes a second working electrode catalytic layer and a second pair of electrode catalytic layers. The second working electrode catalytic layer is disposed between the flow field plate and the second ion exchange membrane and is used to catalyze the electrode reaction on the second working electrode. A second catalyst receiving cavity is provided on the side of the second end plate near the second supporting electrolyte chamber plate. The second pair of electrode catalytic layers are disposed in the second catalyst receiving cavity and are used to catalyze the electrode reaction on the second pair of electrodes.

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