Regulating Surface Electronic Structure Of Pd-based Electrocatalysts And Investigation Of Their Properties

As the anode reaction of direct fuel cell,organic molecular electrooxidation has great applicable potential and enormous efforts are devoted to the development of highly active and stable catalysts.Pd is one of the most potential catalyst to substitute Pt for small organic molecule oxidation reaction.However,Pd-based catalysts still suffer low initial activity and selectivity,inferior anti-poison and stability in small organic molecule oxidation reaction.The application of nanomaterials provides a hopeful choice to solve the above problems.Extensive studies focus on the relationship of Pd-based catalysts’ surface structure and catalytic activity experimentally and theoretically.But we still lack the systematical research on the relationship of electronic structure-reactivity-catalytical activity.It is controllable and cleanable for electrochemical synthesis of Pd-based catalysts,which could reduce the impact of strong adsorbed species on the surface electronic structure of Pd.Therefore,we use square-wave potential and sequential processing to synthesize Pd-Au catalysts.The surface modification and inducing surface strain are applied to regulate the surface electronic structure of Pd-Au catalysts,which change the reactivity of surface on the adsorbed strength.We also have electrochemical catalytical tests for all synthesized catalysts.Finally,the relationships among electronic structure,surface reactivity and catalytical activity are established respectively on Au modified tetrahexahedral Pd nanocrystals and Au@Pd core-shell nanoparticles.This work gives an expanded insight into the structure-reactivity relationship of Pd-Au catalysts,and provides some reference for rational design of new catalysts.The main conclusions are as follows:1.Au/THH Pd NCs are synthesized by modifying Au on surface evenly with various coverage(θ).The results of Cyclic Voltammetry(C V),COpd stripping and in situ FTIRS suggest that the surface reactivity of Pd increases with the θAu.XPS and DFT calculation find that the electronic structure of Pd also has changed(negative shift of 0.15 eV in the binding energy of Pd,up-shift of 0.18 eV in d-band center of Pd).Notablely,the significant variety in electronic structure occurs when θAu is lower 0.4.2.The catalytic activity and stability of Au/THH Pd NCs for EOR have a volcanic trend with θAu.The best one is Au/THH Pd NCs(θAu=0.3 8)whose peak current density(12.7 mA cm-2)is around 4 and 20 times than that of THH Pd NCs and commercial Pd black,respectively,in terms of Liner Sweep Voltammetry(LSV)under the same conditions.In situ FTIRS reveals that the modification of Au promotes the oxidation of ethanol to acetate.We believe that increasing θAu could lift the d-band center and rise the surface reactivity of Pd,which enhances the partial oxidation of ethanol.But the too higher surface reactivity of Pd would result in inferior anti-poison of Pd to inhibit EOR.3.The Au@Pd NPs are deposited on the surface of glassy carbon electrode by square-wave potential and CV with various segments.HRTEM and mapping mode prove the core-shell structure of Au@Pd NPs.ICP-OES is applied to confirm the ratio of Au and Pd.The results of XPS show that binding energy of Pd shifts positively(about 0.21 eV)with the increase of thickness of Pd shell(approximately 1.8 to 5.3 nm),indicating the change of electronic structure of Pd.Also,the variation of hydrogen adsorption area and COad behavior suggest the decrease of surface reactivity of Pd with thicker Pd shell.The change of catalytic activity of Au@Pd NPs for FAOR corresponds with that of.surface reactivity of Pd:the higher reactivity(the thinner Pd shell)means the higher catalytic activity under the given condition(the thickness of Pd shell is approximately 1.8 to 5.3 nm).The peak current density of Au@Pd:.2 NPs is 6 times higher than that of Pd black in terms of LSV.The current of Chronoamperometry(CA)tests of Au@Pd:.2 increases 7 times than that of Pd black.Better anti-poison of Au@Pd1.2 is ascribed to the better catalytic performance.Considering the relaxed strain in Pd shell,we believe that the thicker Pd shell reduce the relaxed strain,then lead to lower surface reactivity and inferior anti-poison.As a result,the catalytic activity and stability for FAOR decline.4.The surface modification and constructuring core-shell nanoparticles are effective ways to regulate the surface electronic structure of Pd-based catalysts.CO and H are used as adsorbed probe to study the surface reactivity.Conbin,ing the electrooxidation performance of Pd-based catalysts,the relationship among the d-band center of Pd(electronic structure),binding strength of adsorbed species(surface reactivity)and activity toward EOR and FAOR(electrocatalytical performance)are illuminated.The connection between electrocatalytical materials and electrocatalytical process,which further expands the applicaton of d-band center theory and strain effect in small molecular electrooxidation,casts new light on the development of active electrocatalysts.