Low-temperature Catalytic Combsution DCM on Ce and Mn Oxides and Performances Tuning

Author:Zhang Xin Hua

Supervisor:huang hai feng

Database:Doctor

Degree Year:2019

Download:6

Pages:149

Size:13120K

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Chlorinated volatile organic compounds(CVOCs),as a kind of ubiquitous air pollutants,are harmful to air quality and human beings since they are important precursors of photochemical smog,greenhouse effect and ozone depletion in stratosphere.Thus,it is urgent to develop new materials,new techniques and new ways to solve the effluent of CVOCs.Catalytic combustion is one of most perspective ways for the problem.Indeed,the key technology for this method is to develop new catalytic materials.As environmental friendly materials,the oxides of Ce and Mn could be used as catalysts in CVOCs combustion.However,the general oxides of Ce and Mn showed low catalytic performances in CVOCs combustion,thus,it is necessary to enhance the activity and selectivity of oxides of Ce and Mn in CVOCs combustion by tuning their natures.Based on this view,a series of CeO2 nanoparticles with different morphologies(nano-rods,nano-cubes,nano-polyhedrons)were synthesized by a simple hydrothermal way,and further doped with Mn in their specific exposed crystal planes.Using dichloromethane(DCM)combustion as a model reaction,the activity and chlorinated products selectivity of the above mentioned catalysts were investigated.Based on the same way,the activity and chlorinated products selectivity of DCM combustion on OMS-2 catalysts doped with different types of transition metals and different amount of Ce using reflux heating methods,respectively,were also investigated.At last,the relationship of activity and structures of these as-prepared catalysts were revealed combined various characterization methods.The main results in this paper were thus summarized as follows:(1)Significant morphology effects were observed when DCM oxidation on CeO2nano-catalysts.CeO2 nano-rods showed good activity in DCM combustion,the T50 and T90 were 278°C and 323°C,respectively;however,the CeO2 nano-cubes showed bad activity in DCM combustion,the T50 and T90 were 359°C and 413°C,respectively.The products distribution wasn’t identical when DCM oxidation on CeO2 nano-catalysts with different morphologies.For HCl,the CeO2 nano-polyhedrons showed good selectivity while CeO2 nano-cubes showed bad selectivity.The yield of trichloromethane(TCM)and carbon tetrachloride(CTC)was restrained when DCM conversion on CeO2 nano-rods,while the CeO2 nano-polyhedrons were benefit to these two chlorinated products.(2)Thanks to versatile characterization methods,the correlation of the specific exposed crystal planes of CeO2 nano-catalysts and the activity and selectivity of DCM conversion on CeO2 nano-catalysts were also confirmed.The oxygen vacancies concentration and the amount of surface absorbed oxygen was influenced by the polarity of the specific exposed crystal planes and the close packing styles of Ce and O atoms,which further influenced the activity and selectivity of DCM oxidation on CeO2nano-catalysts with different morphologies.(3)The performances of CeO2 nano-rods in DCM combustion were regulated by two ways of Mn doping,that is,in-situ doping way and ex-situ doping way,respectively.The yields of HCl,TCM,and CTC were increased when DCM oxidation on these Mn doped CeO2 nano-rods catalysts,although their activities decreased a little as compared with the pure CeO2 nano-rods.Based on ex-situ doping way,the activity and selectivity of CeO2 nano-rods with different amount of Mn doped in DCM oxidation were further investigated,and the same results were obtained.The CeO2 nano-rods with 4 wt.%of Mn doped showed the best selectivity in HCl selectivity among all investigated samples.The activities of CeO2 nano-rods in DCM oxidation were hardly changed when the amount of doped Mn was above 6 wt.%.The catalysts characterizations showed that the surface oxygen vacancies concentration and absorbed oxygen were increased on the CeO2 nano-rods with Mn doped,however,the morphologies,specific exposed crystal planes,and fluoric type structures of the investigated samples were unchanged with Mn doped.However,the doped Mn were easily aggregated on the surface of CeO2 nano-rods,resulted in the decrease of BET surface areas and lost part of active sites,eventually,the activity of CeO2 nano-rods with Mn doped in DCM oxidation dereased,meanwhile,the increased yields of HCl,TCM and CTC may be related to the increased acidity,also the increased hydrogen-bond interactions between surface bonded chlorinated species on(111)crystal planes and OH-groups on CeO2 nano-rods with Mn doped.(4)The natures of CeO2 nano-cubes were also regulated by the same ways as the above mentioned.Unlike the CeO2 nano-rods,the activities of CeO2 nano-cubes in DCM oxidation were increased with Mn doped.For CeO2 nano-cubes with Mn doped in the ex-situ way,the yields of TCM and CTC were also restrained.Based on this way,the activities and selectivity of CeO2 nano-cubes with different amount of Mn doped in DCM oxidation were also investigated.The results showed that the activities of the investigated samples in DCM oxidation firstly climbed up then declined with the increasing amount of Mn doped.The CeO2 nano-cubes with 4 wt.%of Mn doped showed the best activity,and the yields of TCM and CTC were also restrained on this catalyst.The activities of CeO2 nano-cubes in DCM oxidation were little changed once the amount of Mn doped was above 8 wt.%.The morphologies and fluoric types of CeO2 nano-cubes were both unchanged with Mn doped,and Mn entered into the lattices of CeO2 through isomorphous substitution.The doped Mn can easily aggregated on the surface of CeO2 nano-particles,resulted in the decrease of BET surface areas of the samples.The concentration of surface oxygen vacancies and absorbed oxygen on CeO2nano-cubes was also changed with Mn doped.The highest oxygen vacancies concentration was observed on the sample with 4 wt.%amount of Mn doped.The activities of CeO2 nano-cubes in DCM oxidation were decreased with Mn dopping amounts further increased since this could be resulted in the significantly decrease in BET surface areas,surface active sites and pores of the catalysts.(5)The activity and selectivity of OMS-2 in DCM oxidation were regulated with different types of transition metals doped.The OMS-2 with Sr and Cr doped showed good activity and HCl selectivity in DCM oxidation,respectively.The yields of TCM and CTC were also restrained when DCM combustion on Cr doped OMS-2,however,a considerable amount of TCM and CTC were observed on Sr doped OMS-2.The OMS-2 with Fe and V doped not only showed detrimental activity in DCM oxidation,but also bad selectivity in HCl and TCM when DCM combustion on Fe doped OMS-2.However,a good HCl selectivity was observed on the V doped sample.The yields of CTC couldn’t be ignored when DCM combustion on OMS-2 with Cu and Ce doped,respectively.The nano-rod morphologies and cryptomelane structures of the transition metals doped OMS-2 were confirmed by the characterization methods.The BET surface areas was influenced by the aggregated transition metals on the surface of OMS-2,and the surface areas of the samples were not consistent with the activities of transition metals doped OMS-2 in DCM combustion,however,the Mn-O lattices in OMS-2 were influenced by the doped transition metals as verified by Raman spectra.The bond force constants of Mn-O in the transition metals doped OMS-2 were calculated according to Raman feature shifts of Mn-O band,which highly consisted with their orders of activities in DCM oxidation.(6)The natures of OMS-2 were also regulated with different amount of Ce doped and used in DCM combustion.The activities in DCM oxidation were decreased a little when OMS-2 doped with Ce,however,their products selectivity was quite different according to the amount of Ce doped.For HCl,OMS-2 with 30 wt.%amount of Ce doped showed good selectivity versus the bad selectivity of OMS-2 with 10 wt.%and40 wt.%amount of Ce doped,respectively.However,the yields of TCM and CTC were restrained when DCM combustion on the latter two catalysts.The nano-rods morphologies and crypotomelane structures were confirmed on all OMS-2 with Ce doped by the catalysts characterization ways.The surface areas and pore volumes were decreased with the increase amount of Ce doped,and the calculated Mn-O bond force constants were basicly consisted with their activities in DCM combustion as mentioned above.