Preparation And Properties Of Core-shell Structure Encapsulating Au-based Catalyst

Due to the excellent catalytic activity and reaction selectivity,nano Au-based catalysts are widely used in many catalytic reactions such as reduction of nitro compounds,selective oxidation of alcohols,water vapor shift reaction,and low-temperature oxidation of CO.The ultra-fine active gold nanoparticles are in a thermodynamically unstable state,and the ultra-high surface energy results in the agglomeration and sintering of the gold particles,leading to the passivation and deactivation of the gold.In addition,during the reaction in the liquid phase,the loss of active substances is also a factor that must be considered to maintain the catalytic activity of gold nanoparticles.Compared with the characteristics of traditional supported Au-based catalysts that are easy to lose during the reaction,the core-shell structure gives full play to its structural advantages and provides a stable reaction site for the active center of Au particles.In the work,this thesis aims at the problems of sintering,agglomeration and loss of supported Au nanoparticles in the catalyst process,and makes full use of the advantages of core-shell structure by constructing an efficient catalytic system.Two kinds of Au-based catalysts with core-shell structure were synthesized by different preparation methods.Based on the liquid-phase oxidation reaction of benzyl alcohol,the structure-activity relationship inside the catalyst was deeply explored,and the specific reaction mechanism was obtained.The details are as follows:1.SiO₂@Co₃O₄/Au@m-SiO₂ catalyst with complete core-shell structure based on SiO₂ was prepared by several effective synthesis steps.The morphologies and structures of the catalysts were characterized by SEM,TEM,BET,XPS,XRD,it can be found that the outer layer of the core-shell structure can effectively prevent the loss of gold nanoparticles,and also provided a stable reaction site for the catalysts.In this process,we studied the effects of multiple factors on the target reaction by controlling a single variable.The experimental results showed that under the optimal conditions（the catalyst dosage,reaction temperature,reaction time and O₂ flow rate were 40 mg,160℃,6 h,60mL/min,respectively）,the conversion rate of the target reaction reached 58%and the selectivity was as high as 82%.Meanwhlie,the performance of the catalyst didn’t significantly decrease after many cycles,which was due to the core-shell structure effectively prevented the loss of AuNPs and the provision of multiple active reaction interfaces.In the end,based on a series of characterization results and related theoretical analysis,a possible mechanism was proposed to illustrate a potential reaction pathway for benzyl alcohol oxidation.2.The CeO₂/Au@CeO₂-MnO₂ catalyst with core-shell structure was derived by in-situ growth of hollow CeO₂spheres,which was prepared by template-free method.In this work,the sandwich hollow structure catalyst was realized by interfacial redox reaction without any surface modification or use of surfactant.Due to the synergy between oxides and Au nanoparticles,the material owned excellent conversion and selectivity for oxidation of benzyl alcohol（BzOH）.Using the method of controlling variables,we explored the influence of catalyst quantity,reaction time,oxygen flow rate and reaction temperature on the reaction,and obtained the best reaction conditions.When the catalyst dosage of 40 mg,reaction time of 6 h,oxygen flow rate of 60 mL/min,reaction temperature of 160℃,65%conversion of benzyl alcohol and 90%selectivity of benzaldehyde were achieved.At the same time,due to the strong limiting effect of the core-shell structure on AuNPs,the catalyst showed good reusability and stability in multiple cycle experiments.In the end,the possible reaction mechanism was put forward to illustrate the specific process of BzOH oxidation.