Synthesis Of Palladium-based Nanocatalysts Promoted By Ceria And Their Electrocatalytic Performance For Alcohol Elecrooxidation

Direct alcohol fuel cells?DAFCs?has attracted widespread attention due to the high energy density,more available fuels,the safety of the storage and transportation of the fuel,simple structure and etc.Pd is cheaper and more abundant than Pt.Furthermore,Pd exhibits excellent performance for alcohol electrooxidation in alkaline medium.In recent years,Pd-based has been a very promising anode catalyst.Nevertheless,the performance of Pd-based catalyst for alcohol electrooxidation need to be improved furtherly in order to realize the commercialization of DAFCs.As a cheap and important rare earth oxide,ceria has been widely used in the fields of catalysis,electrochemistry,fuel cells and etc.,which comes from its unique physical and chemical properties.CeO₂ has not only excellent mechanical properties,but also abundant oxygen vacancy on its surface related to surface defects and high oxygen storage capacity?OSC?,which can promote electron transfer process in catalytic reaction and enhance the resistance of noble metal catalyst to CO poisoning.It is widely used as a cocatalyst to improve the activity and staility of catalyst in recent years.In this thesis,we focused on the enhancement effect of CeO₂ on the alcohol electrooxidation on Pd-based electrocatalysis.Based on the controllable preparation of Pd and PdAg alloy nanocatalysts,the influence of the addition of CeO₂on the performance of Pd-based electrocatlysts for alcohol electrooxidation is investigated by changing the carbon carrier.Specific research contents and main results obtained are as follows:?1?The sucrose as carbon source was pre-carbonized and activated by KOH to synthesis three-dimensional porous carbon?3DPC?with a three-dimensional porous structure.Pd/CeO₂/3DPC catalyst was prepared by deposition-precipitation method using cerium ammonium nitrate as the cerium source and Na₂PdCl₄ as the precursor.Electron microscopy results show that the addition of CeO₂ helps to improve the dispersion of Pd nanoparticles on the surface of the support.The electrochemical test results show that Pd/CeO₂/3DPC catalyst exhibits higher electrocatalytic activity for the oxidation of ethylene glycol and ethanol in alkaline media as compared with Pd/3DPC and commercial Pd/C catalyst.Furthermore,Pd/CeO₂/3DPC exhibits higher catalytic performance in ethylene glycol than that in ethanol.At the same time,electrocatalytic stability test results show that Pd/CeO₂/3DPC has excellent stability for the electrooxidation of ethylene glycol.?2?CeO₂/C composite supports with different CeO₂ loadings was synthesized using deposition-precipitation method using cerium nitrate as the cerium source and Vulcan XC-72 as the carbon support.After that,silver nanoparticles?Ag NPs?was loaded in situ on the surface of the composite support due to the decomposition reaction of silver formate at room temperature.Then the PdAg/CeO₂/C catalyst was successful-lly synthesized through the galvanic replacement reaction.Electrochemical test results show that PdAg/30CeO₂/C?the theoretical mass fraction of CeO₂ on carbon is30%?has excellent activity and stability for the electrocatalytic oxidation of ethylene glycol in alkaline media.At the same time,the apparent activation energy of the catalyst for the electrochemical oxidation of ethylene glycol was reduced after the addition of CeO₂ in it.?3?A red blood cell-like nitrogen-doped carbon material?N-C?was synthesized by polymerizing of dopamine on polymethyl methacrylate?PMMA?and carbonizing.Then PdAg/CeO₂/N-C catalysts with different CeO₂ loadings were synthesized by deposition-precipitation and galvanic replacement.The surface of as prepared N-C is rich in N atoms,which proves that dopamine can be used as an effective nitrogen source for the N-doping of carbon materials.Electrochemical test results show that an appropriate doping amount of CeO₂ in as-prepared catalysts can improve significantly its performance for the electro-oxidation of ethanol,while an excessive doping amount of CeO₂ will reduce their performance.The order of electrocatalytic activity of several different catalysts for ethanol electro-oxidation in alkaline media is:PdAg/30CeO₂/N-C>PdAg/C?PdAg/50CeO₂/C>PdAg/70CeO₂/C.Among them PdAg/30CeO₂/N-C shows the best electrocatalytic activity and stability.