Study On The Synthesis Of Modified Ta₂O₅ Supported Pt Catalysts And Performance For Methanol Oxidation

In order to solve the contradiction between the growing demand for energy and the shortage of energy,new alternative energy and clean production technologies have been extensively studied.Direct methanol fuel cell（DMFC）is considered as one of the efficient and environmentally friendly power generation technologies because of its high energy conversion rate,green and pollution-free,and long-term sustainable operation.However,DMFC has not achieved large-scale commercial application due to the low catalyst activity,poor chemical stability,easy poisoning and high production cost.In recent years,the use of metal oxides as supports to support Pt catalysts has attracted extensive attention.Especially,due to high dielectric constant,excellent chemical and thermal stability,stable existence in both acidic and alkaline solutions,Ta₂O₅ has promising applications in fuel cells.But compared with the traditional carbon supports,the poor conductivity of pure Ta₂O₅ is not conducive to electron transfer in electrocatalytic reaction.In addition,fewer active centers lead to low Pt dispersity.In this paper,Ta₂O₅ was modified to study its catalytic performance for methanol oxidation.The specific research work includes the following aspects:（1）Using transition metal（ruthenium）doped Ta₂O₅（Ru-Ta₂O₅）as support,and then using NaBH4 liquid phase reduction method to prepare Pt/Ru-Ta₂O₅ series catalysts.It was proved by XRD and XPS that Ru was successfully doped into the Ta₂O₅ lattice.TEM results showed that the Pt nanoparticles with an average particle size of 4.0±0.5 nm were uniformly dispersed on the surface of Ru10%-Ta₂O₅ support.Compared with pure Ta₂O₅ supported Pt nanoparticles,the particle size was smaller and the dispersion was more uniform.The four-probe results also show that Ru doping can significantly improve the conductivity of Ta₂O₅.The catalytic MOR performance of Pt/Ru10%-Ta₂O₅ catalyst was tested by cyclic voltammetry electrochemical test.The forward peak current of methanol oxidation on the Pt/Ru10%-Ta₂O₅ catalyst is 482.2 mA mg1,which are 3.6 times and 2.3 times of Pt/Ta₂O₅（133.50 mA mg-1）and Pt/C catalyst（207.78 mA mg-1）,respectively.（2）Ta₂O₅（NP-Ta₂O₅）was co-doped with non-metallic elements（N,P）,melamine and triphenylphosphine were used as N and P sources,and then Pt/NP-Ta₂O₅ catalysts were prepared by NaBH4 liquid-phase reduction method.XRD and TEM results proved that Pt nanoparticles were successfully supported on NP-Ta₂O₅.Compared with Pt/Ta₂O₅ catalyst,Pt nanoparticles of Pt/NP-Ta₂O₅ catalyst had smaller particle size（4.12±0.5）and more uniform dispersion,which fully demonstrates the positive role of N and P elements in the process of loading Pt nanoparticles onto the Ta₂O₅ surface.XPS results demonstrate that N,P dual elements are successfully doped into the Ta₂O₅ lattice,which can better promote the electrocatalytic performance due to the synergistic effect between the heteroatoms.The electrochemical results show that the forward peak current of methanol oxidation on Pt/NP-Ta₂O₅ catalyst is 439.17 mA mg-1,which are 3.29 times and 2.11 times of Pt/Ta₂O₅（133.50 mA mg-1）and Pt/C catalyst（207.78 mA mg-1）,respectively.In addition,the oxidation onset potential of Pt/NP-Ta₂O₅ is 0.129 V,which is lower than that of Pt/Ta₂O₅（0.257 V）and commercial Pt/C（0.132 V）,indicating that the Pt/NP-Ta₂O₅ catalyst has better catalytic activity.（3）Ta₂O₅ was coated with poly aniline and then heat-treated to obtain Ta₂O₅ coated with nitrogen-doped carbon layer（CNx@Ta₂O₅）,and then Pt/CNx@Ta₂O₅ was prepared by NaBH4assisted ethylene glycol reduction.FTIR and SEM results showed that the PANI coating was successfully carbonized to form CNx@Ta₂O₅.Raman demonstrated that at a carbonization temperature of 800℃,the CNx@Ta₂O₅-800 support had more structural defects as the active sites for dispersed and stabilized Pt nanoparticles.The XPS data also proved that C、N were successfully introduced into the carbon skeleton.TEM and XRD results showed that the Pt nanoparticles of Pt/CNx@Ta₂O₅-800 catalyst have smaller particle size（3.87±0.5 nm）and more uniform dispersion,which fully indicates that the positive role of core shell structure in the process of loading Pt nanoparticles onto Ta₂O₅ surface.The electrochemical results show that the Pt/CNx@Ta₂O₅-800 electrocatalytic methanol oxidation peak current density is the highest（395.83 mA mg-1）when the carbonization temperature is 800℃.In addition,the peak current density of methanol oxidation of Pt/CNx@Ta₂O₅-800 is much better than that of Pt/Ta₂O₅（133.50 mA mg-1）catalyst.These results indicate that the core shell structure is more favorable for improving the electrocatalytic activity of MOR.