Studies Of Surface Structure,Composition And Properties Of Pd-based Electrocatalysts

Direct liquid fuel cells(DLFCs)have attracted considerable attention due to some advantages as,possessing high theoretical energy density and environmental-friendly features,rich sources and easy storage of fuel and so on.But this kind of fuel cell still suffers from the sluggish kinetic process and toxic intermediates especially in their anodic reactions.Although noble metals catalysts show good activities for organicmolecule electro-oxidation(formic acid,methanol,ethanol and etc)reactions,they are not only limited by the problems mentioned above but also face the obstacles of low storage and expensive prices.For pursuing better activities and higher utilization efficiencies of noble metals,rationally tailoring catalysts’ surface structures and tuning their surface compositions are of great significance.Among numerous noble metals,Pd-based catalysts do well in formic acid oxidation reaction(FAOR)and ethanol oxidation reaction(EOR)but their catalytic activities and stabilities remain to be improved for the practical application.Therefore,this thesis focuses on the preparation of Pd-based catalysts with enhanced reactivity for organic-molecule electro-oxidation by controlling the surface structure and composition.The main works are as following:1、We prepared PdPt concave nanocubes on one side of carbon paper(PdPtCNC/CP)by facile electrodeposition.By adjusting the lower limit potential,we can obtain nanocrystals with clear concave structures and high density of nucleation.It has been demonstrated that PdPt-CNC/CP owned high-index facets,such as(310),(410),(510)and(710).According to XPS results,alloying of Pd and Pt made the binding energy of Pd 3d shift more positively(0.38 eV)than pure Pd,indicating the change of d-band energy level.In the PdPt-CNC/CP,the atomic ratio of Pd and Pt was 4:1 by EDS and ICP-MS tests.The PdPt-CNC/CP exhibited enhanced activities on formic acid oxidation(8.72 mA cm-2 and 2.31 A mg(Pd+Pt)-1),which were 4.0 and 10.5 times of those of Pd black)and ethanol oxidation(38.60 mA cm-2 and 10.20 A mg(Pd+Pt)-1),which were 6.9 and 11.4 times of those of Pd black,7.6 and 12.8 times of those of Pt black.After 1800 s’chronoamperometry tests,the activities of PdPt-CNC/CP for FAOR and EOR still showed higher activities.Meanwhile,PdPt-CNC/CP had enhanced selectivity of C1 products in EOR.2、We firstly prepared PdCo tetrahedron(PdCo-TH)catalysts by one-step hydrothermal method.And then a small amount of Ir was doped into PdCo tetrahedron by the same way.The Ir-doped catalysts also had tetrahedron structures and Ir tended to exist on the surfaces by the TEM,ICP,XRD and XPS.XPS results showed that the binding energy of Pd 3d in PdCoIr-TH/C shifted more negatively(～0.1 eV)and the content of Pd(0)increased than PdCo-TH/C.Based on CV tests,PdCo-TH/C and PdCoIr-TH/C had better activities for EOR.At the oxidation peaks,the specific activity and mass activity of Pd9Co1-TH/C(23.00 mA cm-2 and 4.31 A mgpd-1)were 5.9 and 3.7 times of those of Pd-TH/C,10.5 and 6.2 times of those of Pd/C.But the activities of PdCoIr-TH/C at oxidation peaks were smaller than PdCo-TH/C.It’s should be noted that the PdCoIr/C catalysts had better activities at low potentials and Pd9Co1Ir0.1-TH/C was the best one.For example,the specific activity and mass activity of Pd9Co1Ir0.1TH/C were 7.10 mA cm-2 and 1.70 A mgpd-1 at E=-0.35 V,which were 1.4 and 1.8 times higher than those of Pd9Co1-TH/C,4.9 and 3.9 times higher than those of PdTH/C,6.8 and 5.0 times higher than those of Pd/C.The stabilities of PdCoIr/C catalysts were also improved and Pd9Co1Ir0.1-TH/C showed the best stability.Besides,Ir promoted the cleavage of C-C bond in CH3CH2OH based on the results of in situ MSFTIRS and1H NMR.The improved activities and stabilities of PdCo-TH/C and PdColrTH/C may result from these reasons:(a)Co sites on the surfaces can generate moderate OHad spieces so that the performances of these catalysts for EOR are optimized;(b)Ir sites can activate H2O and adsorb OHad easily which not only promote the production of OHad spieces but also benefit to protect adjacent Pd sites from oxidation,so that the reactivity of EOR and the ability of cleavage of C-C bond can be enhanced;(c)At the same time,the change of d-band energy of Pd due to adding Ir might make reaction intermediates more difficult to desorb,which may cause the differences of activities between PdCoIr-TH/C and PdCo-TH/C.Combined with electronic structure effect by alloying,facet structure effect by high-index facets and the Ir-doped surface,Pd-based nanocatalysts’ electrocatalytic activities on FAOR and EOR have been greatly improved.What’s more,the fast preparation of PdPt-CNC on carbon paper without using any surfactant at room temperature and their excellent catalytic activities mean the potential of application in fuel cells and other fields.