Preparation And Electrochemical Catalytic Performance Of Palladium Based Nanosheets

The increase of CO₂ concentration in the atmosphere is the main cause of global warming.Electrochemical reduction of CO₂ into high-value chemicals is a sustainable solution to environmental problems caused by the high concentration of CO₂ in the atmosphere and excessive CO₂ emissions.The fuel cell,which has the advantages of high efficiency and low pollution,is an electrochemical device,which is expected to reduce carbon dioxide emissions in terms of energy acquisition.The nanoscale Pd catalyst can efficiently catalyze the reduction of CO₂ to HCOOH at a low negative potential,and can also promote the efficient conversion of CO₂ to CO at a high negative potential,revealing that the Pd-based catalyst has the characteristics of multifunctional catalysis.In the oxygen reduction reaction,the cathode reaction of a proton exchange membrane fuel cell,the catalytic performance of Pd is second to Pt.The metal Pd has a similar electronic structure to that of Pt and is regarded as the most ideal substitute metal for Pt.Stress effect and ligand effect generated by the alloying of heterogeneous metals will change the bond length of Pd-Pd or Pd-M,affect the d-band center of gravity of the surface Pd atoms,and then change the catalytic performance of Pd.This paper aims to improve the selectivity of carbon dioxide reduction and the efficiency of oxygen reduction reaction.Based on the Pd-based nanosheet structure with abundant active sites and strong bonding strength between the catalyst and the carbon support,adopt the method of organic phase synthesis control the Pd of the electronic structure and bimetallic catalyst structure order degree,enhanced the oxygen reduction activity of Pd catalysts.The high selectivity of formic acid product of Pd at high potential was realized.The main research results are as follows:1)Based on the Pd nanosheet to study the effect of different ratios of Bi doping on its electrochemical performance:The catalyst is expected to improve the product selectivity of Pd at high potential.Using organic palladium compound and organic bismuth compound as precursors,cetyltrimethylammonium bromide(CTAB)as surfactant and surface blocking agent,oleylamine as reducing agent,different ratios of Bi doping Pd nanosheets are prepared at low temperature.Under the action of the organic long chain in CTAB and Br^-,Pd grows in a two-dimensional plane in a sheet-like structure after reduction,with a transverse size of about 150-200 nm and a thinner thickness of about 2-4 nm.The larger surface area of the nanosheets is conducive to electron mass transfer and reduces the migration steric hindrance of the intermediate.The electron density at the surface defects is larger,and it is conducive to increase the anchoring area with the carbon matrix and enhance the bonding strength.Experiments show that in the catalytic carbon dioxide reduction reaction,the doping of Bi is beneficial to improve the selectivity of Pd in the potential of-0.7?0.9V,and the product changes from CO to formic acid.In the catalytic oxygen reduction reaction,the introduction of Bi can increase the activity of Pd nanosheets.The half-wave potential of 5%Bi-doped Pd nanosheets is increased from 0.919 V of pure Pd nanosheets to 0.929 V vs.RHE,with the specific mass activity 7.33 A mg_Pd^-1.After 5000 cycles of testing,the half-wave potential decrease is only 1 m V,the specific mass activity loss is 15.3%,and the stability and activity are better than commercial Pd/C.(2)Preparing Pd Bi bimetallic nanosheet catalysts and exploring the effect of the degree of structural transformation from disorder to order on electrocatalytic performance:By adjusting the synthesis temperature,the bimetallic catalysts of disordered face-centered cubic Pd Bi,partially ordered Pd Bi and fully ordered hexagonal Pd Bi were successfully prepared.The fully ordered hexagonal phase Pd Bi exhibits a completely different crystal structure from face-centered cubic Pd,rhombohedral Bi and disordered Pd Bi alloys and maintains the morphology of nanosheets.Its transverse size is about 150-200 nm and the thickness is about 2-3 nm.There are wrinkles on the surface of the nanosheets.In the carbon dioxide reduction test,the fully ordered Pd Bi has the highest formic acid selectivity and catalytic activity.The Faraday efficiency of formic acid can reach 91.9%at-1.0 V vs.RHE,and over 80%in the potential range of-0.9 V to-1.1 V.In the catalytic oxygen reduction reaction,the catalytic activity and stability of fully ordered Pd Bi are significantly higher than that of disordered Pd Bi alloy and commercialized Pd/C.The half-wave potential is 0.89 V,and the half-wave potential attenuation after 5000cycles is only 1 mV.