Synthesis Of Ceria-based Catalysts With Core-shell And Hollow Structures For Ethylbenzene Oxidation

As an Eco-friendly and rich rare earth metal oxide, ceria has been widely applied in environmental and energy fields. In these fields, cerium oxide can not only be used as active site, but also be utilized as an ideal support when combined with other active sites. In order to improve the thermal stability and avoid the agglomeration of nanoparticles, cerium oxide has been functionalized to synthesize well-dispersed materials, such as core-shell structure with good dispersion and hollow spheres with high specific surface areas. These kind of functional materials can even enhance the activity and stability if combined with other active sites as carriers.Cobalt porphyrin served as a bionic homogeneous catalyst display excellent catalytic activity, but the shortcomings, such as easy to polymerization and difficult to recycle, limits its application. One of the methods to solve the problems is to transform homogeneous catalyst into heterogeneous catalyst via immobilizing cobalt porphyrin on solid carriers. It has been reported previously that the micro environment provided by carrier can greatly improve the stability and anti oxidation of the heterogeneous catalyst. The other one is to synthesize a new material via pyrolyzing cobalt porphyrin precursor in inert atmosphere, which can effectively improve the atom utilization of metal porphyrin. Above all, some new catalysts are prepared in this study and the ethylbenzene oxidation using O2 as green oxidant without solvent is employed to explore the catalytic performance. The detail work is described as following. 1. The core-shell structures that utilized distinct transition metals doped cerium oxide as nuclear and silica as shell are prepared, and the final catalysts are obtained after immobilizing the modified cobalt porphyrin on the surface of supporters through chemical bond. The catalytic performance and stability of the catalysts, namely CoTPP-(MOx/CeO2) (M=Fe, Co, Mn and Cu), are investigated. The result shows that the CoTPP-(CoOx/CeO2) catalyst exhibits the best activity, since the conversion of ethylbenzene reached to 28.5% with 76.8% selectivity to acetophenone.2. The Co-N-C/SiO2@(CoOx/CeO2) catalyst has been successfully fabricated by pyrolyzing the CoTPP-(CoOx/CeO2) at 500? in N2 atmosphere and the performance has been further investigated. The result reveals that the presence of Co-Nx structure which derived from the cobalt porphyrin precursor can enhance the activity of catalyst What's more, the reduced activity of Co-N-C/SiO2@(CoOx/CeO2) is obviously slower than CoTPP-(CoOx/CeO2) after reused 6 times. It is suggest that pyrolyzing cobalt porphyrin to acquire Co-N-C material can largely improve the stability of the catalyst.3. Hollow MnOx/CeO2 spheres with inserted Co-N-C sites (Co-N-C/HMCS) are successfully synthesized by layer-by-layer strategy with cobalt porphyrin as precursor of Co-N-C species. Firstly, the (MnOx/CeO2)@SiO2 core-shell structure is prepared by using SiO2 as hard templates, and then the modified CoTPP is immobilized on the surface of the shell followed by SiO2 coating and pyrolysis treatment at 500? for 1h in N2 atmosphere. Subsequently, the hollow structure is achieved after etched by NaOH solution. As for Co-N-C/HMCS with Co-N-C sites inserted in the shell of the hollow spheres provide a "nanoreactor" environment, and the outstanding catalytic performance achieved in the radical reaction of ethylbenzene oxidation (conversion:44.5%) is owing to the large surface areas and large cages. In addition, the mechanism of catalytic oxidation of ethylbenzene is investigated according to literatures and experiments. It is speculated that the active site of Co-N-C/HMCS are "active" surface oxygen species due to the oxygen vacancy and Co-Nx bond.