The Synthesis Of Pd And PdPt Bimetallic Nanoparticles And The Study Of Their Electrocatalytic Performance

The application of the noble metal platinum as catalyst in fuel cell is greatly limited due to the high cost and scare reserves of platinum.In contrast,palladium with the higher storage and lower cost,as well as its excellent catalytic performance in many catalytic reactions,is a promising catalyst for fuel cell.Besides,because of the synergistic effect between the two elemental compositions and the similar lattice constants of Pd and Pt,it could reduce the consumption of pure Pt and greatly improve the catalytic performance for PdPt bimetallic nanomaterial.In view of above considerations,this dissertation presented a simple one-pot chemical reduction method for the synthesis of Pd nanoparticles with various morphologies.On the basis of this synthetic method,we prepared different morphological and sized PdPt nanoparticles by introducing Pt element and adjusting process parameters,and analyzed the corresponding reaction mechanism,established the growth model.Furthermore,we evaluated the electrocatalytic properties of all kinds of Pd and PdPt catalysts,discussed the potential relationships between the catalytic performance and the shape,size and composition of the catalysts.It is of importance to further improve the synthetic method for noble metal with enhanced performances,research the relevant mechanism,and promote the industrial application of catalyst in fuel cell.The main contents of this dissertation are shown as follows:1.Without the use of seed-mediated method,we prepared different morphological Pd nanoparticles with controllable shape by one-spot chemical reduction approach,and analyzed the relevant growth mechanism.Due to the the oxidative etching function of HCl,the morphological evolution of Pd from cube,truncated cube,cuboctahedron even octahedron would happen through the addition of HCl with different concentrations.In the presence of HCl with appropriate concentration,the corner of cubic Pd would preferentially be oxidized to occur{111}facets.And with the increasing HCl concentration,the oxidative etching function enhanced,the ratio of the growth rate for{111}/{100}gradually decreased,thereby led to the enlargement of{111}facets and the shrinkage of{100}facets,finally the{100}facets with fast growth rate disappeared and Pd grew to octahedral shape.The electrocatalytic results of these Pd catalysts toward formic acid oxidation showed that cubic Pd catalyst had the largest ECSA?10.3m²/g?and the best catalytic activity?345A/g?.And the specific mass activity of cubic Pd was 1.63 times as active as that of commercial Pd black.The catalytic activities of truncated cubic,cuboctahedral and octahedral Pd were lower than that of cubic Pd,which could be ascribed to the slower desorption rate of the intermediate from formic acid oxidation on{111}than that of{100}facets.As observed,the morphologies of different Pd catalysts had changed after cyclic CV measurements.Compared with cuboctahedral and octahedral Pd,the structure of cubic Pd kept a relatively high integrality.The morphological changes of Pd catalysts were mainly ascribed to the dissolution of Pd nanoparticles and the aggregation resulted from the redeposition in the formic acid oxidation.2.Based on the synthetic method of cubic Pd,we prepared PdPt nanoparticles with various core-shell structures using one-pot method with Pt precursors and Pd precursors simultaneously present in the mixture solution.Due to the fast reduction rate of Pd precursor,Pd preferentially nucleated and grew,and subsequently Pt atoms deposited on the surface of central Pd core,finally core-shell structure formed.The distinct core-shell structures could be adjusted by introducing different halide ions.Cl^-ions with relative weak absorption function which failed to change the island growth mode of Pt based on thermodynamically favorable condition,eventually led to the formation of PdPt with dendritic core-shell structure.In the case of Br^-ions with relatively strong adsorption,the lowered surface energy and the slow depositon rate of Pt facilitated the layered growth mode of Pt through the surface diffusion of Pt atoms on the surface of Pd core,and finally the layered core-shell structure of PdPt formed.The stronger adsorption of I^-ions and the resulting fast atomic diffusion function promoted the generation of mesoporous core-shell PdPt.Then we evaluated the electrocatalytic properties of various PdPt bimetallic catalysts with different core-shell structures toward methanol oxidation.The results showed that the mesoporous core-shell PdPt catalyst of 46nm in size with large surface area providing promising active sites had the highest catalytic activity(366A/g_Pd+Pt),which was 3.7 times as active as that of commercial Pt/C.The specific mass activity of the dendritic core-shell PdPt catalyst with the average size of 30nm was 311A/g_Pd+Pt,which was slightly lower than that of mesoporous core-shell one.The specific mass activity of the layered core-shell PdPt catalyst with relatively dense surface was 189A/g_Pd+Pt,which was lower than that of the first two above.However,for the catalytic stabilities among the three kinds of core-shell PdPt bimetallic catalysts above,the layered core-shell one was the best and the mesoporous core-shell one was the worst.Besides,in the case of the same Pt content for PdPt bimetallic catalysts,for example,the mass activities were273A/g_Pd+Pt and 189A/g_Pd+Pt for the dendritic core-shell PdPt catalyst?33.61wt.%?and layered core-shell PdPt catalyst?33.05wt.%?respectively,and the former was as active as 1.4 times that of the latter.Similarly,the specific mass activities were 239A/g_Pd+Ptd+Pt and 133A/g_Pd+Pt for the dendritic core-shell PdPt?22.45wt.%?and layered core-shell PdPt?25.45wt.%?respectively,and the former was 1.8 times as active as that of the latter.All these results displayed the catalytic activities were dependent with the morphologies of PdPt catalysts.3.Through further modifying the synthetic approach of mesoporous PdPt bimetallic nanoparticles,we obtained PdPt catalysts with excellent catalytic performances.First,we prepared hydrangea-like Pd nanoparticles.And the sizes of the pore channels?2⁴nm?and whisker?1²nm?for Pd could be adjusted as a function of reaction time.Subsequently,by employing Pt precursor with different concentrations or the mixture solution of Pt precursor and a certain amount of NaBr,the hydrangea-like Pd_xPt_1-x nanoparitcles with different sizes and composition ratios could be achieved Then we evaluated the electrocatalytic properties of the hydrangea-like Pd_xPt_1-x-x bimetallic catalysts with different sizes and composition ratios toward methanol oxidation.The electrocatalytic results of Pd₈₅Pt₁₅ about 80nm,Pd₇₆Pt₂₄ about 75nm and Pd₅₇Pt₄₃ about 77nm with mass activities 105.6A/g_Pd+Pt,299.9A/g_Pd+Pt and181.0A/g_Pd+Pt respectively toward methonal oxidation,indicated that the catalysts would have high activities in the case of appropriate Pt content.If the content of Pd was too high,the lack of Pt would lead to the slow dehydrogenation process for methanol,finally lowered the oxidation rate of methanol oxidation and reduced the catalytic activity.If the content of Pt was too high,the lack of Pd would result in the slow dehydrogenation of water to form Pd-OH which would further react with Pt-CO to generate CO₂.Due to the slow formation rate of CO₂ by further oxidation of CO,the catalytic activity and stability declined.When Pt contents were almost the same for different Pd_xPt_1-x catalysts,the reaction time of premade Pd had a significant effect on their catalytic activities and stabilities.For example,Pd₇₆Pt₂₄ about 75nm with 3min reaction time for premade Pd,and Pd₇₈Pt₂₂ about 109nm,Pd₇₅Pt₂₅ about 84nm with30min reaction time for premade Pd,the catalytic activities for the three were299.9A/g_Pd+Pt,150.7A/g_Pd+Pt and 124.7A/g_Pd+Pt,respectively.This difference could be ascribed to the whole sizes and the various microstructures of Pd_xPt_1-x,the different sizes of pore channels and whiskers for premade Pd resulted form different reaction times.Overall,because of the large amounts of pore channels and whiskers with several nano size for the hydrangea-like Pd_xPt_1-x catalyst,it faciliated methanol molecules more accessible to the surface of the catalyst.Meanwhile,the large surface area provided more promising active sites for catalytic reaction,thus improved the utilization and catalytic efficiency of catalyst greatly.Obviously,the catalytic activities of these hydrangea-like Pd_xPt_1-x catalysts toward methanol oxidation were composition-dependent,with Pt in 24⁴0atomic%exhibiting the optimum activity and stability.