Study On Microstructure And Catalytic Properties Of Porous Nano-spinel Cobalt Salts

As p-type semiconducting functional materials,cobaltite MCo2O4(M=Zn,Ni,Cu and Fe),a series of binary metal oxides with spinel structure,are suitable for ion diffusion and electron transfer owing to the large amount of interspace residing in the crystal structure.Combined with nano-porous structure,these materials will have porous structure,high specific area and high adsorption performance,which leads to more catalytic active points,making cobaltates as ideal candidates for applications including electrochemical oxidation of glucose and catalyzing the thermal decomposition of ammonium perchlorate.Therefore,preparing porous nanostructured MCo2O4(M=Zn,Ni,Cu and Fe)via simple and practical methods to reduce the cost of production and promote their performance in as-mentioned applications,along with exploring the mechanism of the formation of porous structure and catalysis,have their meaning in both scientific research and future applications.In this work,porous cobaltite MCo2O4(M=Zn,Ni,Cu and Fe)were successfully prepared by solution combustion and solvothermal methods using F127 as soft template,and their performances as the catalysts of electrochemical oxidation of glucose and thermal decomposition of ammonium perchlorate were investigated.Major findings are as following.1.MCo2O4(M=Zn,Ni,Cu and Fe)prepared using solution combustion method were hierarchically-porous structured with high specific area,indicating the successful development of a novel,simple and effective method for preparing porous binary metal oxide.Study on the formation mechanism of porous structures revealed that the ratio between combustion agents and oxidants was a critical factor in the solution combustion method with regard to getting pure phase porous binary metal oxide.2.MCo2O4(M=Zn,Ni,Cu and Fe)were obtained via solvothermal method using F127 as soft temples.These materials had various morphogenesis from sheet-and-rod-liked,rock-liked and tremella-liked.Formation mechanisms of MCo2O4(M=Zn,Ni,Cu and Fe)were further investigated.F127 copolymer went through micro phase separation in the mixed solvent and then self-assemblied under the effects of abundant O-H bonds present in the solution,while diverse structures were formed as the results of different electrostatic interactions between metal cations.After carbonation and removal of F127 through annealing,porous structures were constructed based on the various morphology formed before.All pores were mesopores and the specific area was further increased with values around three times higher than those of cobaltite prepared using solution combustion method.3.After partial substitution of Co cations in Co3O4 by other metal cations,MCo2O4 synthesized using solution combustion method showed significant improvements in the electrocatalytic oxidation of glucose,especially for NiCo2O4,which had the highest sensitivity of 2.100 mA·mM-1·cm-2,broadened sensing range and superior long-term stability among all cobaltite prepared in this work.Prepared via solvothermal method with F127 as template,NiCo2O4 had a specific area three times higher than that of the same type of material prepared using solution combustion method,which offered more catalytic active layer,promoted the catalysis behavior and therefore increased the sensitivity of the biosensor to 2.184 mA·mM-1·cm-2.After 30-day repeated measurement,the sensitivity maintained 93.8%of its initial value,revealing higher stability compared with that given by its counterpart synthesized via solution combustion method(90%of initial value).Further study on the catalysis mechanism indicated that the enhanced oxidation of glucose originated from more M(II)/M(III)redox couples owing to the substitution of Co atoms in crystal structure by other metal atoms in MCo2O4(M=Zn,Ni,Cu and Fe).4.The catalytic performance on thermal decomposition of AP was increased after part of the Co cation in Co3O4 being replaced by other metal cations.FeCo2O4 prepared using solution combustion method,benefiting from its high specific area,showed the best catalytic performance,reducing the high decomposition temperature of pure AP to 181.8?,which was better than currently reported works.With threefold specific area compared with FeCo2O4 obtained using solution combustion method and therefore more active points,that from solvothermal method with F127 as soft template reduced the high decomposition temperature of pure AP to 196.3 ?,showing even better performance.Further study on the catalysis mechanism showed that there were more unfilled d-orbitals after the substitution of Co by other atoms,enhancing the electron transfer based on electron transfer theory,along with the coordination effects in multi-metal oxide,contributed to the higher catalytic behavior on thermal decomposition of AP.