Electrode self-cleaning method in gas-phase sulfur dioxide electrolytic reduction process

Abstract

The invention discloses an electrode self-cleaning method in a gas-phase sulfur dioxide electrolytic reduction process, and belongs to the technical field of sulfur dioxide flue gas resource utilization. In the traditional SO2 electrolysis process, sulfur is adhered to an electrode, so that the effective reaction area of the electrode is reduced, catalytic active sites of the electrode are covered, the efficiency of an electrocatalytic reduction reaction is reduced, and the conductivity and the like of the electrode are further influenced along with continuous accumulation of elemental sulfur on the surface of the electrode, the resistance of the electrode is increased, and the service life of the electrode is prolonged. The service life of the electrode is shortened, and the production cost is increased. According to the method, SO2 gas is intermittently introduced, when SO2 is introduced, SO2 reduction reaction mainly occurs on the surface of the electrode, when SO2 is stopped from being introduced, proton adsorption and elemental sulfur reduction reaction occur on the surface of the electrode, deposition of elemental sulfur on an electrode interface is avoided, and self-cleaning of sulfur on the surface of the electrode is achieved.

Description

Technical Field

[0001] The invention relates to a method for electrolytic reduction of gaseous sulfur dioxide, in particular to a method for self-cleaning electrodes during electrolytic reduction of gaseous sulfur dioxide, and belongs to the technical field of resource utilization of sulfur dioxide flue gas. Background Art

[0002] In the process of preparing H2S by direct electrocatalytic reduction of gaseous SO2, the generated H2S easily undergoes secondary reaction with unreacted SO2 to produce elemental sulfur (2H2S + SO2→3S + 2H2O), and elemental sulfur is very easy to adhere to the surface of the catalytic electrode, which will bring many problems: on the one hand, it will reduce the effective reaction area of ​​the catalytic electrode, cover the catalytic active sites of the electrode, and cause the efficiency of the electrocatalytic reduction reaction to decrease; on the other hand, as elemental sulfur continues to accumulate on the electrode surface, it will have an adverse effect on the conductivity and other properties of the electrode, increase the electrode resistance, shorten the electrode service life, and thus increase the production cost. Therefore, it is urgent to develop a method that can effectively solve the problem of sulfur adhering to the electrode during the electrochemical reduction of gas-phase sulfur dioxide. Summary of the invention

[0003] In view of the technical problems faced in the prior art of direct electrolytic reduction of gas-phase sulfur dioxide due to sulfur adhesion to the electrode, resulting in low electrolytic reduction efficiency, high energy consumption, short electrode service life, etc., the purpose of the present invention is to provide a method for self-cleaning electrodes in the electrolytic reduction of gas-phase sulfur dioxide. The method can avoid the contact reaction between the introduced sulfur dioxide and the generated hydrogen sulfide to generate elemental sulfur by intermittently introducing sulfur dioxide gas. During the period when no sulfur dioxide is introduced, a small amount of elemental sulfur generated on the electrode surface can be reduced to form hydrogen sulfide (S+2H + +2e - =H2S), thereby achieving self-cleaning of the electrode surface, solving the technical problem of sulfur adhesion, and realizing long-term stable electrolysis of sulfur dioxide.

[0004] In order to achieve the technical purpose of the present invention, the present invention provides a method for self-cleaning electrodes during the electrolytic reduction of gas-phase sulfur dioxide. The electrolytic reduction process adopts a two-chamber flow electrolytic cell as the electrolytic system, and the two-chamber flow electrolytic cell includes an anode chamber and a cathode chamber. The anode chamber adopts an inert anode, and the cathode chamber adopts a cathode with the function of electrocatalytic reduction of sulfur dioxide. The cathode chamber and the anode chamber are separated by a diaphragm. Electrolyte is introduced into the anode chamber and sulfur dioxide gas is intermittently introduced between the cathode chambers for electrolytic reduction. The sulfur dioxide gas is highly selectively converted into hydrogen sulfide gas, and is avoided from being converted into elemental sulfur and deposited on the cathode surface.

[0005] In the process of direct electrolytic reduction of gaseous sulfur dioxide, the H2S generated on the electrode surface does not have time to escape, and will easily undergo a secondary redox reaction with the newly adsorbed SO2 on the electrode surface to produce elemental sulfur (the specific reaction formula is as follows: 2H2S+SO2→3S+2H2O), and the generated elemental sulfur is deposited and adhered to the electrode surface. The key to the present invention is to use intermittent sulfur dioxide gas. During the SO2 introduction stage, the electrode surface mainly undergoes a SO2 reduction reaction to generate hydrogen sulfide gas. During the SO2 introduction stop stage, it is mainly beneficial for the hydrogen sulfide gas generated on the electrode surface to escape in time, reduce the contact reaction between hydrogen sulfide gas and sulfur dioxide, and avoid the generation of elemental sulfur. At the same time, the reduction potential is used to reduce the small amount of elemental sulfur deposited on the electrode surface to form hydrogen sulfide (S+2H + +2e - →H2S), thereby achieving self-cleaning of the electrode surface, effectively solving the technical problem of sulfur adhesion on the electrode surface during the direct electrolytic reduction of gas-phase sulfur dioxide, and achieving long-term stable electrolysis of SO2.

[0006] As a preferred solution, the intermittent sulfur dioxide gas process is: 5s~5min as a cycle, and the sulfur dioxide gas introduction time and non-introduction time in each cycle account for 5%~95%: 95%~5%. The period of intermittent sulfur dioxide introduction and the proportion of sulfur dioxide gas introduction time in each cycle are optimized and determined according to the characteristics of the electrode material, reaction rate, etc. For example, the proportion of sulfur dioxide gas introduction time in each cycle can be appropriately adjusted according to the rate of sulfur deposition on the electrode surface during electrolytic reduction. The faster the rate of sulfur deposition, the shorter the proportion of sulfur dioxide gas introduction time in each cycle should be, which is easy for those skilled in the art to think of based on the concept of the present invention.

[0007] As a preferred solution, the electrolytic reduction adopts direct current electrolysis or pulse electrolysis. As a more preferred solution, the voltage range of the direct current electrolysis is 0.1V~10V, and the current range is 0.01A~20A.

[0008] As a preferred scheme, the pulse electrolysis includes pulse current or pulse potential; the pulse period of the pulse current is 5s~5min, and the high current time and low current time in each period account for 5%~95%: 95%~5%; the high current is 0.1~20 A, and the low current is less than 0.1A; the pulse period of the pulse potential is 5s~5min, and the high potential time and low potential time in each period account for 5%~95%: 95%~5%; the high potential is 3~5V, and the low potential is less than 3V.

[0009] As a preferred solution, when the electrolysis adopts pulse electrolysis, the cycle of pulse electrolysis is consistent with the cycle of intermittent introduction of sulfur dioxide gas, that is, when the potential is low or the current is low, the introduction of sulfur dioxide gas is stopped, and when the potential is high or the current is high, the sulfur dioxide gas is introduced. The reaction principle based on the electrolytic reduction of sulfur dioxide gas includes the reactant adsorption process and the process of sulfur dioxide combining with protons to be reduced to hydrogen sulfide gas. More specifically, when a low potential or a low current is applied, the introduction of SO2 is stopped, and proton adsorption mainly occurs on the electrode surface, mainly SO2 and protons are adsorbed on the electrode surface to provide protons for the reduction of deposited sulfur; when a high potential or a high current is applied, SO2 combines with protons to be reduced to H2S, and elemental sulfur is also easy to combine with hydrogen protons to be converted into H2S, thereby utilizing the periodicity of pulse current or pulse potential to realize the reduction process of deposited sulfur and sulfur dioxide, thereby improving the conversion efficiency of SO2 to H2S, reducing energy consumption, and realizing the consumption of deposited sulfur, thereby achieving the purpose of self-cleaning.

[0010] As a preferred solution, the cathode chamber uses a cathode with the function of electrocatalytic reduction of sulfur dioxide, including porous electrodes such as carbon felt and metal mesh, or an electrode loaded with a sulfur dioxide reduction catalytic material, wherein the sulfur dioxide reduction catalytic material includes at least one of a metal element of copper, platinum or palladium, a metal sulfide of copper, platinum or palladium, and a metal selenide of copper, platinum or palladium. The preferred sulfur dioxide reduction catalytic materials are all catalytic materials with high electrocatalytic reduction activity of sulfur dioxide that are well known in the prior art. The electrode can be one of platinum sheet, platinum mesh, nickel foam, platinum carbon paper, iridium oxide carbon paper, titanium felt, titanium sheet, and titanium mesh.

[0011] As a preferred solution, the inert anode is one of platinum sheet, platinum mesh, nickel foam, platinum carbon paper, iridium oxide carbon paper, titanium felt, titanium sheet, and titanium mesh.

[0012] As a preferred solution, the volume concentration of SO2 in the sulfur dioxide gas is 0.1% to 100%, and the balance gas is an inert gas, such as argon, helium, etc.

[0013] The electrolyte in the anode chamber of the present invention can be water or an aqueous solution, and the aqueous solution can contain an electrolyte that improves conductivity. The electrolyte mainly generates hydrogen protons through electrolysis to combine with sulfur anions reduced by the cathode to generate hydrogen sulfide gas.

[0014] Compared with the prior art, the technical solution of the present invention brings the following beneficial technical effects: 1) The stability and service life of the electrode can be significantly improved. The present invention effectively avoids the deposition of elemental sulfur by controlling the intermittent introduction of SO2 ( Figure 1), after long-term operation test, the catalytic activity of the electrode can still be maintained above the initial value, and the voltage / current runs smoothly, which greatly extends the service life of the electrode, reduces the frequency of equipment maintenance, and ensures the continuity of production.

[0015] 2) The method is simple and easy to operate, which is conducive to industrial promotion. On the basis of existing electrolysis equipment, only a simple gas flow control device needs to be added to realize the intermittent introduction of SO2, without the need for complex equipment modification and high investment. In addition, the reaction conditions are easy to control, the technical requirements for operators are not high, and the transformation and application of technological achievements can be quickly realized. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 The electrode surface after electrolysis in Example 1 with continuous SO2 flow and Example 1 with intermittent SO2 flow is shown.

[0017] Figure 2 This is a test diagram of the intermittent SO2 direct current electrolysis stability in Example 3.

[0018] Figure 3 This is a graph showing the relationship between pulse electrolysis voltage and time for comparison between Example 2 with continuous SO2 flow and Example 4 with intermittent SO2 flow.

[0019] Figure 4 This is a test chart of the intermittent SO2 pulse electrolysis stability in Example 5.

[0020] Figure 5 Schematic diagram of an electrolytic cell for electrolyzing gaseous sulfur dioxide to produce hydrogen sulfide. DETAILED DESCRIPTION

[0021] The following examples are intended to further illustrate the present invention, but are not intended to limit the present invention. Corresponding adjustments and improvements made without departing from the concept of the present invention all fall within the scope of protection of the present invention.

[0022] Unless otherwise specified in the following examples, the chemical reagents used are conventional commercially available products and are of analytical grade.

[0023] In the following examples and comparative examples, the electrolysis system is a two-chamber flow electrolysis cell, such as Figure 5 The two-chamber flow electrolysis cell comprises an anode chamber and a cathode chamber, wherein the anode chamber adopts an IrO2 / CP anode, and the cathode chamber adopts a hydrophobic carbon felt cathode, wherein the cathode chamber and the anode chamber are separated by a commercial proton exchange membrane, H2O is introduced into the anode chamber and sulfur dioxide gas is introduced into the cathode chamber, and direct current, pulse current or pulse potential is applied between the anode and the cathode for electrolysis, and the sulfur dioxide gas is catalytically reduced and converted into hydrogen sulfide.

[0024] Comparative Example 1 Experiment on the production of hydrogen sulfide by electrolysis of gaseous sulfur dioxide under direct current potential (without stopping the gas flow): Hydrophobic carbon felt is used as cathode, carbon paper loaded with iridium oxide is used as anode (denoted as IrO2 / CP), proton exchange membrane is used as diaphragm, and the electrode area is 1cm 2 The volume concentration of SO2 was 20%, the balance gas was argon, the gas flow rate was 20 mL / min, the DC electrolysis was 20 min, and the applied potential was 3 V. The Faraday efficiency of H2S was 43.17%, and the partial current density was 116.32 mA / cm 2 .

[0025] Comparative Example 2 Experiment on the production of hydrogen sulfide by pulse potential electrolysis of gaseous sulfur dioxide (without stopping the gas flow): Hydrophobic carbon felt is used as cathode, carbon paper loaded with iridium oxide is used as anode (denoted as IrO2 / CP), proton exchange membrane is used as diaphragm, and the electrode area is 1cm 2 The volume concentration of SO2 is 20%, the balance gas is argon, the gas flow rate is 20mL / min, the pulse electrolysis is 20min, the pulse period is 2min, and the high potential time accounts for 50%. The high potential is 3V for 1min, and the low potential is 2.5V for 1min. The Faraday efficiency of H2S is 55.65%, and the partial current density is 72.87mA / cm 2 .

[0026] Example 1 Experiment on producing hydrogen sulfide by direct current electrolysis of gaseous sulfur dioxide (intermittent ventilation): Hydrophobic carbon felt is used as cathode, carbon paper loaded with iridium oxide is used as anode (denoted as IrO2 / CP), proton exchange membrane is used as diaphragm, and the electrode area is 1cm 2 , SO2 volume concentration is 20%, balance gas is argon, gas flow rate is 20 mL / min. Intermittent ventilation cycle is 4 min, ventilation time accounts for 50%, SO2 on time is 2 min, SO2 off time is 2 min, DC electrolysis is 20 min, applied potential is 3V. The Faraday efficiency of H2S is 75.31%, and the partial current density is 202.92 mA / cm 2 .

[0027] Example 2 Experiment on producing hydrogen sulfide by direct current electrolysis of gaseous sulfur dioxide (intermittent ventilation): Hydrophobic carbon felt was used as cathode, carbon paper loaded with iridium oxide was used as anode (denoted as IrO2 / CP), proton exchange membrane was used as diaphragm, and the electrode area was 1 cm 2, SO2 volume concentration is 20%, balance gas is argon, gas flow rate is 20mL / min. Intermittent ventilation cycle is 4 min, ventilation time accounts for 50%, SO2 on time is 2 min, and SO2 off time is 2 min. DC electrolysis for 20 min, applied current is 100 mA. The Faraday efficiency of H2S is 82.47%, and the partial current density is 82.47 mA / cm 2 .

[0028] Example 3 Stability experiment of producing hydrogen sulfide by direct current electrolysis of gaseous sulfur dioxide (intermittent ventilation): Hydrophobic carbon felt was used as cathode, carbon paper loaded with iridium oxide was used as anode (denoted as IrO2 / CP), proton exchange membrane was used as diaphragm, and the electrode area was 1 cm 2 , SO2 volume concentration is 20%, balance gas is argon, gas flow rate is 20mL / min. Intermittent ventilation cycle is 4 min, ventilation time accounts for 50%. SO2 on time is 2 min, SO2 off time is 2 min. Applied current is 100 mA. After 40 min of electrolysis, the voltage increased by ~0.5V compared with the beginning of electrolysis ( Figure 3 ), sulfur adheres to the electrode surface. The Faraday efficiency of H2S is 77.27%, and the current density is 77.27 mA / cm 2 .

[0029] Example 4 Experiment on pulse electrolysis of gaseous sulfur dioxide to produce hydrogen sulfide (intermittent ventilation): Hydrophobic carbon felt was used as cathode, carbon paper loaded with iridium oxide was used as anode (denoted as IrO2 / CP), proton exchange membrane was used as diaphragm, and the electrode area was 1 cm 2 , SO2 volume concentration is 20%, balance gas is argon, gas flow rate is 20 mL / min. Intermittent ventilation cycle is 2min, ventilation time accounts for 50%. SO2 on time is 1 min, SO2 off time is 1 min, ventilation cycle is consistent with pulse voltage cycle, that is, ventilation at high potential. Pulse electrolysis for 20 min, pulse cycle is 2min, high potential time accounts for 50%. Apply high potential 3V for 1 min, apply low potential 2.5V for 1 min ( Figure 2 ). The Faradaic efficiency of H2S is 87.89% and the current density is 115.09 mA / cm 2 .

[0030] Example 5 Stability experiment of pulse electrolysis of gaseous sulfur dioxide to produce hydrogen sulfide (intermittent ventilation): Hydrophobic carbon felt is used as cathode, carbon paper loaded with iridium oxide is used as anode (denoted as IrO2 / CP), proton exchange membrane is used as diaphragm, and the electrode area is 1cm 2 , SO2 volume concentration is 20%, balance gas is argon, and gas flow rate is 20mL / min. The intermittent ventilation cycle is 1min, and the ventilation time accounts for 50%. The SO2 on time is 30s, and the SO2 off time is 30s. The ventilation cycle is consistent with the pulse voltage cycle, that is, ventilation at high current. The pulse cycle is 1 min, and the high potential time accounts for 50%. Apply a high current of 250 mA for 30s, and a low current of 50mA for 30s. The electrolysis runs stably for 12 hours, and the voltage does not decay ( Figure 3 ). The Faradaic efficiency of H2S is 86.74% and the current density is 130.11 mA / cm 2 .

[0031] Compared with direct current electrolysis, the use of pulse electrolysis of gaseous sulfur dioxide to produce hydrogen sulfide (intermittent ventilation) can achieve long-term and stable electrolysis of gaseous SO2, completely solve the problem of sulfur adhesion to the electrode, and has the potential to expand current density.

Claims

1. A method for electrode self-cleaning during electrolytic reduction of gas phase sulfur dioxide, wherein the electrolytic reduction process uses a two-chamber flow electrolytic cell as an electrolytic system, the two-chamber flow electrolytic cell includes an anode chamber and a cathode chamber, the anode chamber uses an inert anode, the cathode chamber uses a cathode with a function of electrocatalytic reduction of sulfur dioxide, the cathode chamber and the anode chamber are separated by a diaphragm, characterized in that: Electrolytic reduction is performed by introducing electrolyte into the anode chamber and intermittently introducing sulfur dioxide gas into the cathode chamber. The sulfur dioxide gas is highly selectively converted into hydrogen sulfide gas, while being avoided from being converted into elemental sulfur deposited on the cathode surface.

2. The method for electrode self-cleaning in the process of gas-phase sulfur dioxide electrolytic reduction according to claim 1, characterized in that: The process of intermittently introducing sulfur dioxide gas is as follows: one cycle is 5s-5min, and in each cycle, the ratio of sulfur dioxide gas introduction time to non-introduction time is 5%-95%:95%-5%.

3. The method for electrode self-cleaning in the process of gas-phase sulfur dioxide electrolytic reduction according to claim 1 or 2, characterized in that: The electrolytic reduction adopts direct current electrolysis or pulse electrolysis.

4. The method for electrode self-cleaning in the process of gas-phase sulfur dioxide electrolytic reduction according to claim 3, characterized in that: The voltage range of the direct current electrolysis is 0.1V~10V, and the current range is 0.01A~20A.

5. The method for self-cleaning electrodes in the process of electrolytic reduction of gas-phase sulfur dioxide according to claim 3, characterized in that: The pulse electrolysis includes pulse current or pulse potential; The pulse period of the pulse current is 5s~5min, and the ratio of high current time to low current time in each period is 5%~95%:95%~5%; the high current is 0.1~20 A, and the low current is less than 0.1A; The pulse period of the pulse potential is 5s~5 min, and the ratio of high potential time to low potential time in each period is 5%~95%:95%~5%; the high potential is 3~5V, and the low potential is less than 3V.

6. The method for electrode self-cleaning in the process of gas-phase sulfur dioxide electrolytic reduction according to claim 1, 2 or 5, characterized in that: When the electrolysis adopts pulse electrolysis, the cycle of pulse electrolysis is consistent with the cycle of intermittently introducing sulfur dioxide gas, that is, the introduction of sulfur dioxide gas is stopped at low potential or low current, and sulfur dioxide gas is introduced at high potential or high current.

7. The method for electrode self-cleaning in the process of gas-phase sulfur dioxide electrolytic reduction according to claim 1 or 2, characterized in that: The inert anode is one of a platinum sheet, a platinum mesh, a nickel foam, a platinum carbon paper, an iridium oxide carbon paper, a titanium felt, a titanium sheet, and a titanium mesh.

8. The method for electrode self-cleaning in the process of gas-phase sulfur dioxide electrolytic reduction according to claim 1 or 2, characterized in that: The volume concentration of SO2 in the sulfur dioxide gas is 0.1% to 100%, and the balance gas is an inert gas.

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