Surface Modification Of Tungsten Trioxide Microspheres And Their Electrocatalytic Properties For Methanol

Methanol oxidation reaction（MOR）has attracted extensive attention because of its important role in the development of direct methanol fuel cells.Tungsten trioxide（WO₃）is often used as a catalyst carrier for fuel cells because of its good stability in acidic environment.However,when WO₃is used as a catalyst carrier,the application of WO₃in direct methanol fuel cells is limited due to its defects such as poor conductivity and small specific surface area.This paper mainly focuses on two aspects of reducing the band gap of WO₃and increasing its conductivity,so as to improve the electrocatalytic activity of catalyst and reduce the cost of catalyst.Firstly,the first principles are used to screen the doped elements and optimize the doping position,and the microporous structure and electronic structure are adjusted at the same time,to improve the conductivity and promote the transfer of reactants,and achieve the improvement of catalyst activity.Finally,the controllable synthesis strategy of doped tungsten trioxide was studied to further verify the theoretical calculation results.（1）The band structures and local state densities of WO₃,N-WO₃and S-WO₃bulk phases were calculated by first principles.The WO₃band gap decreased obviously after N and S doping.By further calculation of adsorption energy,it was found that it was difficult for N-WO₃（0001）surface to adsorb CO after the adsorption of Pt clusters.In addition,the adsorption energy of CO on the three surfaces was in the order of Pt/N-WO₃-CO>Pt/S-WO₃-CO>Pt/WO₃-CO.Differential charge density and electron localization function were used to analyze the nature of electron interaction between Pt clusters and CO molecules on WO₃,S-WO₃and N-WO₃bulk phase（0001）surfaces.The results showed that the introduction of N and S atoms can greatly promoted the accumulation and redistribution of surface charge,effectively reduced the interaction between CO and Pt clusters,and improved the tolerance of CO to the catalyst.（2）Nitrogen-doped WO₃microspheres with mesoporous structure were prepared by spray drying and calcination and loaded with precious metal Pt on their surfaces.TEM SEM and XRD were used to analyze the morphology and phase distribution of the support and catalyst.The results showed that nitrogen was successfully doped into the microspheres and Pt nanoparticles were evenly dispersed on the surface of the support.The band gap of N-WO₃and WO₃was calculated to be 2.21 e V and 2.52 e V by solid state UV spectroscopy.Then,the methanol oxidation performance of each catalyst was further analyzed by cyclic voltammetry（CV）timing current（It）impedance（EIS）and other testing methods.The results showed that the peak current density of the first methanol oxidation peak of Pt/N-WO₃catalyst was 510m A/mg,which was 1.7 and 2.2 times of that of JM Pt/C（298 m A/mg）and Pt/WO₃（230m A/mg）,respectively.Therefore,Pt/N-WO₃catalyst showed stronger catalytic activity for methanol oxidation.The results of 6000 s timed current test showed that the current density of Pt/N-WO₃catalyst was always higher than that of JM Pt/C and Pt/WO₃,which indicated that nitrogen-doped tungsten oxide microspheres could effectively improve the CO toxicity and stability of Pt to methanol during anodic oxidation.The results indicated that the impedance of Pt/N-WO₃catalyst is much smaller than that of Pt/WO₃catalyst.（3）Sulfur-doped WO₃microspheres with mesoporous structure were prepared by spray drying method and roasting method,and precious metal Pt was loaded on the surface.Finally,the preparation of Pt/S-WO₃catalyst was completed.The band gap of S-WO₃was 2.30 e V and that of WO₃was 2.52 e V by solid state UV spectroscopy.SEM and TEM test results revealed that compared with WO₃microspheres,the structure of S-WO₃microspheres did not change much,and Pt nanoparticles were dispersed on the surface of WO₃microspheres.The electrocatalytic oxidation performance of Pt/S-WO₃catalyst in acidic solution was studied by cyclic voltammetry impedance and chronocurrent method.The oxidation peak current density of Pt/S-WO₃catalyst was 480 m A/mg,which was higher than that of JM Pt/C（298 m A/mg）and Pt/WO₃（230 m A/mg）catalysts.The results showed that Pt/S-WO₃catalyst had higher electrocatalytic activity for methanol oxidation than JM Pt/C and Pt/WO₃catalyst.