Synthesis Of Molybdenum Based Catalyst And Its Performance For The Catalytic Wet Air Oxidation Of Cationic Dye Wastewater

With the rapid development of textile industry,it is urgent to solve such serious environmental pollution caused by dye wastewater with high concentration of organic pollutants generated from those industries.Among all the techniques on wastewater treatment,catalytic wet air oxidation(CWAO)is one of the most widely used method.However,it still requires a relatively high temperature(200-280?)and a pressure(2-8 MPa).To solve the problems such as severe reaction conditions,high costs and energy consumptions,we focused on developing novel catalysts with high efficiency and reusability.In this study,a series of molybdenum based catalysts which were synthesized by molten-salt and hydrothermal methods showed promising catalytic activities on the degradation of dye wastewater under room temperature and atmospheric pressure.The effect of synthesis methods,molar ratios of alkali or transition metal and Mo,synthesis pH values,and calcination temperatures on their crystal structure and morphology were investigated systematically.And we also studied the influence of testing temperatures and catalyst dosages on catalytic performance during CWAO process.Moreover,the reaction kinetic,intermediate,and degradation mechanism were further investigated.Firstly,M2Mo4O13/?-MoO3(M=Li,Na,K)and MMoO4/a-MoO3(M=Mn,Ni)samples with morphology of nanofiber were successfully synthesized by hydrothermal method.The influence of synthesis pH values,molar ratios of M and Mo,calcination temperatures on crystal structure and morphology were investigated as well.M2Mo4O13/?-MoO3 hybrid material showed good catalytic activity and reusability after 5 cycles,and Na2Mo4O13/?-MoO3 hybrid material can be prepared in a much wider pH range(3-6)by adding more Na species.Moreover,MnMoO4/?-MoO3and NiMoO4/?-MoO3 hybrids showed better stability.The removal of dye could be achieved at 99.43%and 99.08%,separately,after 6 cycles.The factors of morphologies of catalysts,calcination temperatures,testing temperatures and catalysts dosages in CWAO process on degradation of dye wastewater were also investigate with Na2Mo4O13/?-MoO3,MnMoO4/a-MoO3 and NiMoO4/a-MoO3 hybrids as catalysts.The results showed that much more active sites have been gained due to its unique morphology of nanofiber,which makes contribution to its excellent catalytic performance.However,catalytic activity decreased with increase of calcination temperature because of the growth of crystallite size.In addition,the increase of testing temperature can speed up the generation of hydroxyl radicals during CWAO process and the increase of catalysts dosage can provide more active sites,which lead to higher degradation efficiency.The optimal condition of testing temperature and catalyst dosage is 30? and 0.33 g/LMoreover,the reaction kinetic,intermediate,and degradation mechanism were further investigated by different techniques.The activation energy and reaction rate constant for using MnMoO4/a-MoO3 and NiMoO4/a-MoO3 hybrids as catalyst are 2190.49 J/mol and 1.88×10-4,2981.15 J/mol and 1.08×10-4,respectively.The existence of hydroxyl radicals was confirmed by ESR analysis,which is crucial for the degradation of dye wastewater.Those free radicals are generated from the interaction between catalysts and adsorbed oxygen and make dye molecules easily to be attacked.In the end of CWAO process,the organic pollutants will be transformed into CO2 and H2O.Finally,the reason why transition metal molybdate/molybdenum trioxide hybrid material shows the best catalytic activity during CWAO process is that the stongest interaction between MMoO4 and ?-MoO3 reduce dissolution of Mo,which enhance the stability of hybrid material.On the other hand,the synergistic effect motivate the oxygen activity during the redox process and also reduce the activation energy,which accelerated the degradation of dye wastewater.