In recent years, economic growth has promoted the vigorous development of medicine, pesticides, chemical production and other fields. At the same time, the output and types of organic compounds have shown explosive growth. Due to the characteristics of organic matter being difficult to degrade, easy to bioaccumulate, and highly biotoxic, discharging into water bodies will pollute the environment and threaten human health. The electrocatalytic oxidation method has the advantages of high efficiency and no secondary pollution in degrading organic pollutants in industrial wastewater, which has prompted more scholars to devote themselves to the research of using electrocatalytic oxidation technology to treat organic pollutant wastewater.
DSA (Dimens-naOyStableAnode) is a coating anode made by coating some metal oxides with electrocatalytic activity on the surface of a titanium substrate. It has the advantages of stable anode size, good electrocatalytic activity, low working voltage, and long life. It has been widely used in treating organic pollutant wastewater). RuO2 has excellent oxygen evolution activity and electrocatalytic activity, and is often selected as the main catalytic component of DSA coating. However, the Ti-Ru binary coating anode still has problems in application, such as the high price of precious metals, resulting in excessively high electrode preparation costs; RuO2 itself has poor stability, and the substrate titanium is easily oxidized, which weakens the adhesion of the coating and causes it to fall off, resulting in electrode inactivation). These defects make the Ti-Ru binary coated titanium anode unable to meet the needs of organic pollutant wastewater treatment. Doping Ir, Mn, Sn, Ta, Pb, Zr and other inert components in the ruthenium coating to prepare a multi-component composite oxide coating can improve the stability of the active oxide, reduce the amount of precious metals, save the cost of electrode preparation, and improve the electrode catalytic efficiency, making it more suitable for treating organic wastewater. This paper reviews the research and application progress of multi-component ruthenium coated titanium anode in the field of organic pollutant wastewater treatment in recent years.
1. Ruthenium-based titanium anode
Ruthenium-based titanium anode is made by coating an active layer of ruthenium oxide on the surface of a titanium substrate. Its preparation methods include: thermal decomposition, electrodeposition, magnetron sputtering, sol-gel, etc. RuO2 has low overpotential for chlorine and oxygen evolution, which is conducive to the occurrence of catalytic chlorine and oxygen evolution reactions. However, the Ti-Ru binary oxide coating anode has the disadvantages of high preparation cost, short life, and unsatisfactory catalytic activity when used for catalytic degradation of difficult-to-degrade substances. Therefore, it is necessary to add a third or even a fourth component to the coating to improve the electrode performance and make it suitable for treating organic pollutant wastewater.
1.1 Ternary ruthenium-based titanium anode
The ternary ruthenium-based titanium anodes currently under study include Ru-Ir, Ru-Mn, Ru-Sn, Ru-Ta, Ru-Pb, Ru-Zr, etc.
1.1.1 Ru-Ir et al. prepared Ti-based binary (Ti-Ru) and ternary (Ti-Ir-Ru) oxide coating electrodes by sol-gel method, and compared their coating morphology and service life. It was found that the crack morphology of the ternary coating was more uniform and more active under lower potential conditions, but under high potential conditions, the load transfer capacity of the ternary coating would be reduced. Adding IrO2 is conducive to the oxygen evolution reaction, and is conducive to extending the service life of the electrode, and can prevent the decomposition reaction of the active site of RuO2. Ti-based ternary (Ti-Ir-Ru) oxide coating electrodes were prepared, and it was found that increasing the number of coatings within a certain range is conducive to the occurrence of chlorine evolution reaction on the electrode surface, but too many coatings will inhibit the production of chlorine. When the number of coatings reaches 6 times, the chlorine evolution effect is the best.
1.1.2 Ru-Mn The variable valence property of manganese makes its oxide have excellent chemical and electrochemical activity. Adding Mn to the ruthenium coating is conducive to improving the electrocatalytic activity of the ruthenium titanium anode. We used thermal decomposition to prepare Ti/RuO2-MnO2 electrodes. The results showed that Mn is a suitable additive for RuO2 electrodes. A rutile solid solution was measured on the electrode surface, which is conducive to the occurrence of oxygen and chlorine evolution reactions. When the molar contents of Ru and Mn are 70% and 90%, respectively, the electrode stability and electrocatalytic activity are optimal.
1.1.3 Ru-Sn Adding tin (Sn) to the ruthenium coating is beneficial to improving the conductivity of the electrode. We used thermal decomposition to prepare Ti-based ternary (Ru-Sn-Ti) oxide coating electrodes under different calcination temperatures, and found that: at 400°C, the electrode life reaches the maximum when the Ru content is 55%-60%. At 500°C, the electrode life reaches the maximum when the Ru content is 30%-55%. The service life of RuSn-Ti electrodes is significantly higher than that of Ru-Ti electrodes. We studied the effect of changing the solvent (from HCL to isopropanol) on the Ti/RuO2-SnO2 electrode and concluded that using isopropanol as a solvent is conducive to solvent evaporation, which can reduce the number of coatings, reduce the loss of tin, improve the stability of the coating, and increase the electrochemically active surface area. We studied the effect of the coating composition on the oxygen evolution reaction of the Ti/RuO2-SnO2 electrode and concluded that the coating composition has no effect on the mechanism of oxygen generation. As the SnO2 content increases, the electrocatalytic activity of the electrode surface increases. After a large amount of oxygen is precipitated, the surface charge drops sharply and the electrode surface morphology collapses. It is believed that Sn activates the electrode, resulting in a decrease in the stability of the RuO2+TiO2 electrode.
1.1.4 Ru-Ta Adding Ta to the coating makes the dispersion of RuO2 more uniform, thereby improving the electrode activity. We prepared Ti/RuO2-Ta2O5 electrodes with different Ru and Ta contents. The results showed that the introduction of Ta2O5 in the RuO2 coating can prevent the corrosion/oxidation of RuO2 under higher potential conditions. Under more stringent operating conditions, the electrode performance is closely related to the coating morphology and structure. The passivation of the Ti matrix surface is the main reason for electrode deactivation. As the calcination temperature decreases, the electrode stability decreases. The OER mechanism is related to the loading in the coating. It is generally believed that the oxide loading is positively correlated with the number of coatings, so increasing the number of coatings can extend the electrode life.
We prepared Ti/RuO2-Ta2O5 electrodes under different calcination temperatures and found that when the calcination temperature is low, the crystallinity of RuO2 in the coating is low. As the calcination temperature decreases, the RuO2 crystal structure changes from crystalline to amorphous. RuO2 amorphizes at 260°C. The amorphization of RuO2 can increase the active surface area of the oxygen evolution reaction, reduce the oxygen evolution potential, and improve the electrode electrocatalytic activity.
1.1.5 Ru-Pb isotope Ti/Ru0.3Pb0.7-xTiO2 (0≤X≤0.7) electrodes were prepared by thermal decomposition of inorganic salts. Scanning electron microscopy (SEM) data showed that the lead oxide content gradually increased, the mud cracks on the coating surface gradually disappeared, and Pb segregated to form island-like protrusions. Adding Pb is beneficial to improving coating stability, extending service life, and improving OER catalytic activity. The improvement of OER catalytic activity can be attributed to three factors: ① Change in oH bond length; ② Electrodes with high Pb content have more compact coating structures and small spacing between oxide particles, which is beneficial to electron transmission; ③ The role of morphological effects.
1.1.6 Ru-Cz zirconium dioxide (ZrO2) has attracted much attention because it has both acidity and alkalinity as well as oxidation and reduction properties. He Jianfu et al. prepared Ti/RuO2-ZrO2 electrodes at 450°C by thermal decomposition. The results showed that the coating surface was smooth, with many evenly distributed cracks, and no RuO2 particles were observed to precipitate. Adding Zr is beneficial to improving the electrode's selectivity for gas precipitation, and has a positive effect on the electrode's life and catalytic activity. When the Zr content is 60%, the electrode performance is optimal.
1.2 Quaternary ruthenium titanium anode
Cerium (Ce) is a rare earth element with relatively active chemical properties. It can be used as a pore initiator to make the oxide grains disperse more evenly, thereby increasing the active surface area and improving the oxygen evolution activity. At the same time, it can improve the electrode conductivity and oxygen evolution capacity. Goudarzi et al. used thermal decomposition method to prepare Ce-modified Ti/RuO2(0.5)-CO3O4(0.5) electrode at a calcination temperature of 400℃. The results show that after the addition of rare earth Ce, the active surface area, roughness and electrode voltammetric charge of the coating inside and outside are significantly increased, and the apparent activation energy of the oxygen evolution reaction is significantly reduced. Ruan Qin et al. prepared Ti/RuO2(0.5)-CO3O4(0.7-x)-CeO2(x)(0≤X≤0.7) electrodes with different Ce contents by sol-gel method. The results show that doping with an appropriate amount of CeO2 is beneficial to reducing the grain size and increasing the active surface area. The CeO2 content is strongly correlated with the electrocatalytic activity. When the CeO2 content reaches 40%, the catalytic activity and voltammetric charge are optimal.
We studied the effect of rare earth Nd doping on Ti/RuO2-Co3O4 anode and concluded that the addition of Nd can improve the electrode performance in two aspects. On the one hand, it can refine the surface grains of the coating, make the crystal form fuller, and enrich RuO2, thereby enhancing the catalytic activity; on the other hand, it can enhance the bonding force between the substrate and the coating, prevent electrode passivation and deactivation, and extend the electrode life. We prepared Ti/RuO2-CeO2-Nb2O5 electrode, and studied its surface electrocatalytic activity and the mechanism of oxygen evolution and chlorine evolution. It was found that due to the synergistic effect of Ru and Cv oxides, the intrinsic electrocatalytic activity of the oxygen evolution reaction is completely determined by electronic factors. When the CeO2 content is high, the global electrocatalytic activity is the highest, which is caused by both electronic and geometric factors, while the real electrocatalytic activity is determined by electronic factors. The chlorine evolution reaction has nothing to do with the composition of the oxide coating. When no Nb2O5 is added, the electrocatalytic activity of the chlorine evolution reaction reaches the best. Adding CeO2 will greatly increase the porosity. Therefore, when the CeO2 content is high, the coating is less stable. Nb2O5 can make the coating more compact, so adding Nb2O5 is beneficial to improve stability.
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Nicole
Company: Baoji Jimiyun Dynamic Co., Ltd
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