Catalytic Wet Air Oxidation(CWAO) Of Phenol On Cerium Based Catalysts

Catalytic wet air oxidation(CWAO)is an effective technique to treat industrial wastewater,especially for highly toxic phenolic streams,where bio-treatment is often less practical.Ceria is a common rare-earth metal oxide,which is widely used as active center,promoter,and support in CWAO of phenol.Since phenol oxidation is carried out in aqueous condition,the effect of water has to be taken into consideration.Theoretical studies have shown that water can be converted into stable hydroxyl groups on oxygen vacancies from the surface of ceria.This effect is expected to have large impact on CWAO of phenol.However,it is not reported in the literature.Herein,this thesis start from the effect of oxygen vacancies on phenol oxidation,and studied the physic-chemical properties of cerium based catalysts on CWAO of phenol.First,based on the current knowledge about the application of cerium based catalysts on CWAO of phenol,we have developed nano-structured CeO2 with different amount of oxygen vacancies,and used for phenol oxidation.It was found that CeO2 nanorods contain greater amount of oxygen vacancies and resulted higher catalytic activity toward phenol oxidation.Characterizations including.TEM,XPS,TPR have proved that the presence of oxygen vacancies can promote the transition between Ce4+ and Ce3+,which can increase the redox properties of CeO2 and make catalyst surface more oxidizing.In order to further investigate the effect of oxygen vacancies on CWAO of phenol,we have used various methods to treat CeO2 nanorods,and synthesized catalysts with different amount of oxygen vacancies and Ce3+ concentration.It was found that CeO2 nanorods treated in H2O2 with sonification can dissolve surface Ce3,increase Ce3+ fraction,decrease the band gap of CeO2 nanorods,and create more oxygen vacancies.The oxygen vacancy clusters can promote the transition between Ce4+ and Ce3+,and improve the oxidizing ability of CeO2 nanorods surface.Meanwhile,H2O2 sonification treatment also increased the concentration of surface hydroxyl group,which can improve the catalytic activity.CeO2 nanorods treated with various methods displayed different catalytic activities on CWAO of phenol.The results suggested that catalytic activity is directly related to the concentration of Ce3+.CeO2 nanorods treated in H2O2 with sonification showed highest Ce3+ fraction and phenol oxidation activity.This is due to the increased oxygen mobility by oxygen vacancies.Doping is another technique to increase the amount of oxygen vacancies on ceria.Meanwhile,the interactions between metal oxides can also improve catalytic activity.Mn was doped into nano-ceria to produce MnOx-CeO2 nanorods and nanocubes.The results showed that MnOx-CeO2 nanorods have more surface oxygen vacancies,which improved the redox properties of MnOx-CeO2 and created more active oxygen species.It was also discovered that MnOx-CeO2 nanorods can form more homogeneous solid-solution than Mnx-CeO2 nanocubes,and more Mn have entered Ce lattice to form-Mn-O-Ce-bonds.Different Mn/Ce interactions have also caused different electron transfer ability between Mn and Ce,where MnOx-CeO2 nanorods contain higher amount of Mn4+ and MnOx-CeO2 nanocubes have greater Mn2+concentration.These characteristics enabled MnOx-CeO2 nanorods to have the highest catalytic activity amount all the catalysts studied.Deactivation of catalyst is a big challenge for CWAO of phenol.The above catalysts were recycled and used for several times.It was found that CeO2 and MnOx-CeO2 catalysts deactivated after the first reaction.By summarizing the literature,we have synthesized Pt/CeO2-TiO2 catalysts through hydrothermal impregnation method.The results suggested that the amount of Ce loading can directly affect the activity and stability of Pt/CeO2-TiO2 catalyst.When Ce loading is 3 wt.%,the catalyst has highest activity and stability,where the TOC conversion remains 91%after 5 consecutive reactions.Different characterizations revealed that 3 wt%of Ce gives smallest Pt particles,which improves the utilization of Pt and enables Pt/CeO2-TiO2 catalyst with the highest oxidation ability.Furthermore,the reduction temperature of the bulk Ce4+ is directly related to the amount of Ce loading.This implies that lattice oxygen mobility varies with different Ce loading,and 3 wt.%of Ce is the optimum loading for Pt/CeO2-TiO2 catalyst.This can provide enough oxygen transfer,while also avoid Pt to be over-oxidized.The elemental analysis also showed that 3 wt.%of Ce gives the lowest carbon deposition,which prevented the coverage of the active center and resulted high catalytic stability.