In Situ Synthesis Of MnO₂/C Catalysts And Its Research On Catalytic Degradation Of Urea Process Wastewater

It is expected that the global urea market capacity will stabilize at about220 million tons by 2020.The high production of urea will inevitably lead to about 110 million tons of process wastewater,which contains about 0.5-2 wt%urea.If urea process wastewater is directly discharged without being treated,which is not up to the emission standard,it will cause the waste of urea and water resources and aggravate the burden of the ecological environment as well.At present,thermal hydrolysis method is generally used to treat urea process wastewater at high temperature and pressure conditions.Therefore,it has high equipment and energy consumption.In addition,there are plentiful methods under the experimental research stage,which are not suitable to industrial treatment of urea process wastewater.However,heterogeneous catalytic degradation is expected to be a promising method for treating urea process wastewater owing to its advantages of low energy consumption and easy separation.Metal or metal oxides were mostly employed to catalyze urea hydrolysis on heterogeneous catalytic degradation methods.However,in order to ensure the excellent performance and reduce the preparation cost of catalysts for promoting industrial applications,the loading catalysts became a solution to above problems.In our work,MnO₂/C catalysts were synthesized in situ loading method and applied to degradate urea wastewater.Factors such as the synthesis conditions and processing conditions were investigated to explore the effect on structure during the preparation of catalysts.And the structure-activity relationships of catalysts were further studied by relevant characterizations.The following are the specific results:1?Weakening alkalinity of solution and increasing synthesis temperatures resulted in a conversion on crystal structure from?-MnO₂ to?-MnO₂,making stability improved.Increasing KMnO₄ solution concentrations and prolonging synthesis time made grain sizes increase structure more regular.However,catalytic activities were impaired when excessive MnO₂ stacked on the surface of carrier.Increasing calcination temperatures made catalyst structure more regular and catalytic activity reduce at a certain extent.However,the specific surface area of catalysts would drop sharply and new substance Mn₃O₄ would be formed at high calcination temperature,making catalytic activity sharply decrease.Prolongating alcination time made grain sizes increase and structure more orderly.Both reducibility and catalytic activity reduced at the same time.2?Combined the molar ratios of Mn³⁺/Mn⁴⁺,the shifts of lattice oxygen position,the contents of surface adsorbed oxygen and AOS of Mn on catalyst surface,the changable trend of oxygen vacancy density was found through calculation and comparison,indicating that the more vacancy defects,the better catalytic activity on urea hydrolysis.3?In situ loading?-MnO₂/C displayed optimal activity with 0.5 mol/L KMnO₄-urea solution,75oC synthesis temperature,8 h synthesis time,400oC calcination temperature and 3 h calcination time.Urea could not be detected in hydrolyzate at 165oC for 150 min.Under the same mass equivalent of MnO₂,urea degradation rate over MnO₂/C was evidently higher than that of pure?-MnO₂ for 30 min.4?In the stability test,the catalyst structure tended to be regular,resulting in a slight decrease on catalytic activity.After 2nd reuse,catalytic activity tended to be stable.Kinetic study showed that MnO₂/C catalyst accelerated urea hydrolysis process and significantly reduced the activation energy.A possible mechanism was proposed on urea hydrolysis over MnO₂/C based on the experimental and characterization results.