Preparation Of Cobalt-Based Double Metal Catalysts And The Application On Degradation Of Pollutants

At present,water pollution has become one of the major threats in the world and many countries are faced with a lack of clean water.The water is polluted by many harmful substances,especially non-degradable organic compounds released from various chemical,dye and pharmaceutical industries.At present,many different methods have been developed to degrade pollutants,including osmosis,adsorption,membrane filtration and ion exchange.Compared with other technologies,advanced oxidation technology（AOPs）has been widely used in the removal of organic pollutants because of its high efficiency,low cost and fewer by-products.Among them,persulfate activation technology is an advanced oxidation technology to produce sulfate radical（SO₄^-·）,which has higher oxidative potential,longer half-life period,better selectivity and a wider range of p H.Hence,SO₄^-·based AOPs has been an ideal method to degrade various pollutants in aqueous solutions.In addition,SO₄^-·is mainly produced through the activation of peroxymonosulfate（PMS）by catalyst.It has been reported that the homogeneous reaction of PMS catalyzed by Co²⁺shows high efficiency in pollutant removal.However,the homogeneous reaction is easy to cause secondary contamination of metal ions,and it is difficult to reuse the catalyst.Therefore,in order to reduce the concentration of metal ions in solution and improve the reuse rate of catalyst,heterogeneous cobalt-based catalyst has a better application prospect.Herein,heterogeneous cobalt-based double metal catalysts had been successfully prepared and its catalytic performance had been systematically studied for activating PMS to degrade pollutants.The details are as follows:1.Firstly,We successfully synthesized Prussian blue analogues（M-Co Co PBA）containing different rare earth metals by chemical precipitation method,and a series of cobalt-based double metal oxides doped with rare earth metals were formed by air pyrolysis with PBA as precursor,and their catalytic performance as heterogeneous catalysts was systematically studied for activating PMS to degrade tetracycline（TC）.It was found that Er-Co₃O₄had the strongest catalytic activity on PMS activation and TC degradation.After reaction for 4 min at 25℃,0.30 g/L PMS and 0.15 g/L Er-Co₃O₄,the TC removal rate in Er-Co₃O₄/PMS system could reach 82%,and the catalyst had good cyclic stability.Free radical quenching and EPR experiments showed that singlet oxygen（¹O₂）,SO₄^-·and·OH were generated in Er-Co₃O₄/PMS system,and these three active substances were all involved in TC degradation.In addition,compared with Co₃O₄,Er-Co₃O₄significantly enhanced the catalytic activity of PMS activation and TC degradation,due to the deterioration of crystallinity and more lattice defects of the catalysts with Er doping.2.Secondly,we used isothermal titration calorimetry（ITC）system to study the thermodynamics of the activation of PMS by Co₃O₄and Er-Co₃O₄catalysts in the temperature range of 298.19 K to 323.19 K,respectively.The rate constants（k）of the activation process at different temperatures were obtained through real-time monitoring of the thermal effects of the activation process and related calculations.Then,the activation energy（E_a）,activation entropy（ΔS^≠）,activation enthalpy（ΔH^≠）and Gibbs free energy（ΔG^≠）of the reaction were obtained by the transition state theory.In addition,the results of thermodynamics study showed that E_a,ΔH^≠,ΔS^≠andΔG^≠of Er-Co₃O₄were lower than those of Co₃O₄.Therefore,the rate constant of catalyst activation PMS was significantly improved,and then the degradation rate of TC was obviously accelerated.3.Finally,we successfully prepared cobalt-based ZIFs containing different transition metals and protected them externally with SiO₂coating.Subsequently,the precursors were vulcanized by hydrothermal method to get a series of SiO₂-coated double metal sulfides（M-Co S_x@SiO₂）,and their catalytic performance as heterogeneous catalysts to activate PMS for MB degradation was systematically studied.The experimental results demonstrated that the MB removal rate of Fe Co S_x@SiO₂/PMS system can reach 97%at reaction condition of 20 min,25℃,0.60 g/L PMS and 0.15 g/L Fe Co S_x@SiO₂.The results of radical quenching and EPR experiments indicated that ¹O₂,SO₄^-·and·OH were the main reactive oxygen species in Fe Co S_x@SiO₂/PMS catalyst system for degrading MB.In addition,the presence of polyvalent sulfur was conducive to the recycling of Co（II）/Co（III）and Fe（II）/Fe（III）.The results of inductively coupled plasma Emission spectrometer（ICP-OES）have also proved that SiO₂shells in Fe Co S_x@SiO₂can effectively reduce metal ion leaching during the reaction.